JP2004528763A - Polarization mode dispersion compensation in optical transmission media - Google Patents

Abstract

The present invention provides a method and apparatus for compensating for polarization mode dispersion (PMD) without perturbing a laser source in an optical transmission system. The present invention compensates for PMD by substantially transferring a sufficient portion of the optical signal of an optical transmission system to a single primary polarization state (PSP) of the system. As a result, each write pulse of the data stream is not substantially mixed with a temporally adjacent write pulse or bit period.
[Selection diagram] FIG.

Description

【００３３】
電界ベクトルは一般に楕円偏光し、即ちＥ_ｘとＥ_ｙは双方とも非ゼロであり、ある期間にわたってＥ_ｘとＥ_ｙの楕円経路を追従する。線形及び円形の偏光は、電界ベクトルが楕円ではなくてそれぞれ直線又は円を時間的に描くので、楕円偏光の状態を縮退する。全ての考えられる偏光を表すための１つの便利な方法は、ポアンカレ球による。
【００３４】
図１を参照すると、偏光状態のポアンカレ球の表現１００が示される。球面上の任意の緯度は、任意の楕円偏光を表し、赤道１０１において直線偏光が表され、極１０３において円偏光が表される。この表現において、球面上の経度の１度は、偏光軸の０．５度の物理的回転を表す。偏光の左右像は２つの半球において変化し、上側の半球では右回りの偏光であり、下側の半球では左回りの偏光である。更に、各経度は、偏光の楕円の半長軸に対する固定方位角を表す。
【００３５】
この表現において、光伝送システムのＰＭＤは、ポアンカレ球上のＰＭＤベクトル（ベクトルΩ）１０２によって表され得る。ＰＭＤベクトル（ベクトルΩ）１０２の方向は、システムのＰＳＰの１つを表すが、ベクトルの大きさはＤＧＤの半分である。任意のサンプル光信号の偏光状態（即ち、偏光監視装置に対する入力偏光状態）は、２つのＰＳＰの線形的な組み合わせである。入力偏光ベクトル１０４（例えば、ストークスベクトル等）を用いて光信号偏光状態を表すことができる。１つのＰＳＰにおける光信号エネルギーの相対的な量は、cos^２（２θ）により与えられ、もう一方はsin^２（２θ）によって与えられ、ここで（２θ）は、入力偏光ベクトル１０４とＰＭＤベクトル（ベクトルΩ）１０２との間のポアンカレ球上の角度である。入力偏光ベクトル１０４がＰＭＤベクトル１０２に沿って下がる場合、第１次に対する全光信号エネルギーは、伝送システムの１つのＰＳＰ内にある。それに対応して、第１次に対する光信号のエネルギーは他のＰＳＰに存在しない。結果として、別のビット期間の他のＰＳＰ（例えば、遅いＰＳＰ）と重なる可能性があり、それによりデータの損失、曖昧性、又は汚染を生じる１つのビット期間の１つのＰＳＰ（例えば、速いＰＳＰ）にエネルギーは存在しない。ＰＭＤに起因する異なるビット期間の異なるＰＳＰの潜在的なオーバーラップは、本明細書においてＰＭＤ拡散と呼ばれる。
【００３６】
また、ＰＭＤベクトルを用いて光伝送システムの各コンポーネント（例えば、光ファイバ部分、光学要素、カプラー、合波器、スイッチ、ルータ等）も表すことができ、共通の座標系におけるこれらのベクトルの合計は、システムの全ＰＭＤベクトルである。理解されるように、システムの全ＰＭＤベクトルは、一般に伝送システムの長さに沿って変化し、時間と共に変化する。例えば、光信号源から１ｋｍの全ＰＭＤベクトルは、一般に信号源から１０ｋｍのものと異なる。同様に、任意の１つの位置における全ＰＭＤベクトルは、例えばシステムのコンポーネントにおける温度及び／又は応力の変化に起因して時間と共に変化する可能性がある。従って、理解されるように、全ＰＭＤベクトル又はシステムのＰＭＤベクトルは、特定のシステム位置及び時間におけるＰＭＤベクトルを指す。
【００３７】
図２Ａを参照すると、本発明によるＰＭＤを補償する一実施形態の概略的なベクトル図が示される。本発明の方法は、光信号エネルギーの十分な部分量を光伝送システムの単一のＰＳＰへと移すことである。本発明の偏光制御装置及び遅延要素からの寄与のない光伝送システムのＰＭＤベクトルは、ベクトル（ベクトルΩ_LINK）２０２によって示される。円形部分２０３は、ベクトル（ベクトルΩ_LINK）２０２と入力偏光ベクトル２０４によって画定される平面にあるポアンカレ球の部分を表す。一実施形態において、本発明の方法は、結果として生じるＰＭＤベクトル（ベクトルΩ_TOTAL）２０８が入力偏光ベクトル２０４と実質的に同じであるアライメントを有するように、偏光制御ベクトル（ベクトルΩ_PC）２０６をベクトル（ベクトルΩ_LINK）２０２に追加することによって、表され得る。ここで、入力偏光ベクトル２０４は、新しいシステムのＰＭＤベクトル（ベクトルΩ_TOTAL）２０８上に実質的にある。結果として、光信号エネルギーの十分な部分量は、システムの１つのＰＳＰへ移動し、それにより第１次に対するＰＭＤ拡散を実質的に補正することによってＰＭＤの影響を補償する。
【００３８】
図２Ｂを参照すると、光エネルギーの十分な部分量を単一のＰＳＰへ移動させる効果が示される。左側の２５０上のプロットは、ＰＭＤ補償前の各ＰＳＰ２５２、２５４の光信号エネルギーを示し、右側２５１上のプロットは、本発明によるＰＭＤ補償後の各ＰＳＰの光信号エネルギーを示す。ＰＳＰ２５２、２５４を表す波形と「ゼロ」の線２５３との間の面積は、波形によって表されるＰＳＰのエネルギーに比例する。図２Ｂの実施形態において、本発明によって提供されるＰＭＤ補償は、速いＰＳＰ２５４からの光信号エネルギーの十分な部分量を遅いＰＳＰ２５２へ移す。しかしながら、理解されるように、光信号エネルギーの全てを単一のＰＳＰに移すことは、本発明にとってあまり重要ではない。それどころか、顕著なＰＭＤ拡散を生じさせるのに不十分なエネルギーの部分量は、ＰＭＤ補償後の速いＰＳＰ２５４に残っているエネルギーによって、右側のプロット２５１に示されるようなＰＳＰに残ることができる。
【００３９】
理解されるように、新しいシステムのＤＧＤ、ＤＧＤ_TOTALは、一般にゼロでない。例えば、図２Ａのベクトルの長さは、各ベクトルに関連したＤＧＤに比例する。従って、図２Ａに示された実施形態の場合、結果として生じるシステムのＤＧＤは、以下のようになる。
【００４０】
【数２】

【００４１】
ここで、ＤＧＤ_LINKはベクトル（ベクトルΩ_LINK）２０２に関連したＤＧＤであり、ＤＧＤ_COMPは補償ベクトル（ベクトルΩ_PC）２０６に関連したＤＧＤである。一実施形態において、ＤＧＤ_COMPはデータストリームの１ビット期間よりも大きく、好適にはＤＧＤ_COMPは、入力偏光ベクトル２０４上に実質的にあるシステムのＰＭＤベクトル（ベクトルΩ_TOTAL）２０８の生成を容易にするためにＤＧＤ_LINKより大きい。
【００４２】
本発明の一態様において、偏光制御ベクトルの追加によって表される物理的プロセスは、偏光制御装置及び遅延要素により提供される。偏光制御装置及び遅延要素は、単一の偏光制御ベクトル、或いは２つ又はそれより多い波長チャネルのそれぞれに対する別個の偏光制御ベクトルを提供することができる。一実施形態において、偏光制御装置は、偏光コントローラと遅延要素を含む。遅延要素は、ＤＧＤ、ＤＧＤ_COMPを提供し、偏光コントローラはポアンカレ球上において光信号に対してＤＧＤ_COMPの方位を変更する。遅延要素は可変ＤＧＤを提供してもよく、又は好適には実質的に固定されたＤＧＤを提供する。偏光制御装置と遅延要素は、単一の集積化光コンポーネント、或いは２つ又はそれより多い光コンポーネントからなることができる。例えば、一実施形態において、偏光制御装置は、２つの光コンポーネント、即ち可変液晶（「ＬＣ」）偏光コントローラと偏光保存ファイバとからなり、可変ＬＣ偏光コントローラは偏光コントローラとして働き、ＰＭＦは遅延要素として働く。
【００４３】
偏光制御ベクトルに選択される方位は、システムのＰＳＰ特性に基づいて決定される。偏光特性は、光伝送システムの光信号の少なくとも一部分を含むサンプル光信号から突きとめられる。一実施形態において、本発明は、サンプル光信号の２つの異なる（好適には、必ずというわけではないが、直交する）偏光成分間に位相遅延を導入し、干渉信号を生成するように２つの偏光成分に干渉を起こさせる。各偏光成分が一般にシステムの２つのＰＳＰの重ね合わせであるので、結果としての干渉信号は、ＰＳＰ間の自己相関成分と相互相関成分を含む。
【００４４】
本発明の偏光監視とＰＭＤ補償の方法は、光伝送システムの２つ又はそれより多い波長チャネルに実質的に同時に適用することに適する。一実施形態において、本発明は、サンプル光信号の２つの異なる（好適には、必ずというわけではないが、直交する）偏光成分間に位相遅延を導入し、干渉信号を生成するように２つの偏光成分に干渉を起こさせる。干渉信号は、各波長チャネルの干渉信号を得ることを容易にするために、スペクトル的に連続した波長サブ帯域へ分散される。
【００４５】
複数の位相遅延において得られた干渉信号を用いて、ＰＳＰの相対的な振幅と偏光状態及びそれらの間のＤＧＤを突きとめることができる。２つの偏光成分間の位相遅延は（例えば、遅延の全波を通じて）変化するので、干渉信号の強度は周期の一部分を通して正弦曲線的に変化し、観測される周期の一部分は、光伝送システムのＤＧＤ、即ちＤＧＤ_LINKに依存する。結果として、干渉信号の強度Ｉは、固定測定周波数ω_０に関して、単位時間における位相遅延ｄの正弦曲線的関数として次のように表され得る：
Ｉ＝Ｉ_０＋Ｃcos（dω_０）＋Ｓsin（dω_０） 式（３）
式（３）の正弦曲線的信号は、係数Ｉ_０、Ｃ、及びＳについて解かれ得る。これらの係数から、サンプル光信号の偏光特性とベクトル（ベクトルΩ_LINK）を求めることができる。
【００４６】
それに対応して、干渉信号が２つ又はそれより多い波長サブ帯域に関して得られる実施形態において、各波長サブ帯域の干渉信号の強度Ｉ（ω）は、位相遅延の正弦曲線的な関数として次のように表され得る：
Ｉ（ω）＝Ｉ_０＋Ｃcos（dω）＋Ｓsin（dω） 式（４）
ここで、ωは関連する波長サブ帯域に対応する周波数である。式（４）の正弦曲線的な信号は、係数Ｉ_０、Ｃ、及びＳについて解かれ得る。更に、第１次のＰＭＤの極限において、ＣとＳの係数も周波数の正弦曲線的な関数であり、次のように表され得る：
Ｃ（ω）＝Ｃ_０＋Ｃ_ｃcos（τω）＋Ｃ_ｓsin（τω） 式（５）
Ｓ（ω）＝Ｓ_０＋Ｓ_ｃcos（τω）＋Ｓ_ｓsin（τω） 式（６）
ここで、τは波長チャネルの光信号のＤＧＤに関連した時間遅延である。任意のデータ集合に関して、これらの式は、ＤＧＤのτ、及びＳ（ω）とＣ（ω）の係数について解かれ得る。これらの係数から、各波長チャネルのＰＳＰ特性を求めることができる。
【００４７】
別の実施形態において、本発明は、第２の一連の位相遅延を導入する前にサンプル光信号の偏光成分に関する偏光軸の回転方位を変更する。この冗長性は、装置が取得されたデータから全ての必要な偏光情報を計算できない場合を取り除く。２つ又はそれより多い回転方位において得られた結果として生じた干渉信号は、ＰＳＰの自己相関と相互相関に追加の情報を提供する。一実施形態において、本発明は、２つ又はそれより多い回転方位のそれぞれについて（１）サンプル光信号の２つの偏光成分間に位相遅延を導入し、次いで（２）任意の回転方位に対して干渉信号を生成するように２つの偏光成分に干渉を起こさせる。この実施形態の１つのバージョンにおいて、干渉信号は２つの異なる回転方位に対して得られる。１つのバージョンにおいて、第１の回転方位は、偏光軸の０°の回転（即ち、回転しない）によって生じ、第２の回転方位は、偏光軸の４５°の回転によって生じる。しかしながら、理解されるように、軸の正確な回転方位は本発明にとってあまり重要ではなく、むしろ非縮退回転方位の任意のセットが使用され得る。また、各回転方位に対して結果として生じる干渉信号は、式（３）と同じ関数形態を有する位相遅延の正弦曲線的な関数としても表され得る。
【００４８】
更に、位相遅延を導入する前にサンプル光信号の偏光軸の回転方位を変更する方法は、２つ又はそれより多い波長チャネルに実質的に同時に適用することに適する。一実施形態において、２つ又はそれより多い回転方位のそれぞれに関して、本発明は、サンプル光信号の２つの偏光成分間に位相遅延を導入し、任意の回転方位に対して干渉信号を生成するように２つの偏光成分に干渉を起こさせる。次いで、干渉信号はスペクトル的に連続した波長サブ帯域へ分散され、任意の回転方位に関して各波長チャネルの干渉信号を得ることを容易にする。また、各回転方位φに対して結果として生じる干渉信号は、式（４）と同じ関数形態を有する位相遅延の正弦曲線的な関数としても表され得る。
【００４９】
例えば、φ＝０°及びφ＝４５°のような２つの回転方位を用いる場合、各波長サブ帯域に対する結果としての干渉信号は、以下のように表される。
【００５０】
【数３】

【００５１】
これらの正弦曲線的な信号は、各波長サブ帯域に関して、Ｉ_０ ^０、Ｃ^０、Ｉ_０ ^４５、Ｃ^４５、及びＳ^４５について解かれ得る。更に、第１次のＰＭＤの極限において、Ｃ^φとＳ^φの係数は、正弦曲線的な横断周波数でもあり、以下のように表され得る。
【００５２】
【数４】

【００５３】
任意のデータセットに関して、これらの式は、ＤＧＤτ、及びＳ（ω）とＣ（ω）の係数について解かれ得る。これらの係数から、各波長チャネルのＰＳＰ特性を求めることができる。
【００５４】
例えば、波長チャネルの光信号のＰＳＰ特性は以下の式から求められ得る。
【００５５】
【数５】

【００５６】
ここで、Ｉ_inputは、監視装置への入射強度を表し、例えば、半波により分離された２つのディザ設定に関して検出された強度を合計することにより、又は一連の測定値の対の強度を合計することにより求められ得る。監視装置におけるＰＭＤベクトルは、Ｅ_ｌｘ、Ｅ_ｌｙ、及びεによって与えられる主偏波状態の方へ実質的に向けられ、一方、ベクトルの長さはＤＧＤτにより求められる。
【００５７】
別の態様において、本発明は、ＰＳＰ特性の測定を容易にする装置を提供する。種々の実施形態において、装置は偏光監視装置と偏光状態生成器を含む。本発明による偏光監視装置の種々の実施形態は、本発明の方法に関する種々の実施形態の更なる説明に関連して以下に説明される。
【００５８】
図３Ａと図３Ｂを参照すると、種々の実施形態において、本発明による偏光監視装置は、回転子３０２、位相遅延生成器３０４、３０５、干渉計３０６、波長分波器３０８、及び検出器３１０を含む。図３Ａと図３Ｂに示されるように、一実施形態において、サンプル光信号３０１は回転子３０２を通過し、その回転子３０２によって、サンプル光信号の偏光楕円の回転方位が位相遅延生成器３０４、３５４の光軸に対して変更されることを可能にする。適切な回転子は、入射の楕円形を実質的に維持しながら、光信号の偏光軸を回転させる（偏光回転子）及び／又は位相遅延生成器の光軸を回転させることができる任意の要素を含む、
【００５９】
例えば、適切な回転子は、以下に限定されないが、ファラデー回転子、速波軸が方位角において所望の回転角度の半分だけ整列または分離された２つの切り換え可能半波長板、及び速波軸が所望の回転角度の半分だけ分離された２つの可変位相リターダを含む。別の実施形態において、回転子は、位相遅延生成器の光軸を回転させる。係る位相遅延生成器は、例えば位相遅延生成器、又は切り換え可能な光軸を有する位相遅延生成器を物理的に回転させる要素を含むことができる。
【００６０】
他の実施形態において、本発明の偏光監視装置は、回転子を含まず、干渉信号は、サンプル光信号の偏光軸の２つ又はそれより多い回転方位において得られない。例えば、サンプル光信号の偏光軸が、可変リターダを含む位相遅延生成器の光軸に沿って存在しない場合、回転子は必要ない。しかし、実際にサンプル光信号の偏光軸が位相遅延生成器の光軸に沿って実質的に線形に偏光される場合、サンプル光信号の２つの偏光成分間に位相遅延は追加されず、この問題に対処するために、回転子が位相遅延生成器の前に追加されてもよい。
【００６１】
図３Ａと図３Ｂを再び参照すると、一実施形態において、サンプル光信号は回転子３０２から位相遅延生成器３０４、３５４へと送られ、その位相遅延生成器は、サンプル光信号の一方の偏光成分を他方に対して遅延する。他方に対して一方の偏光成分を遅延することは、例えば偏光成分を異なる長さの光路を介して、又は可変複屈折要素を介して伝播させることにより達成され得る。種々の実施形態において、位相遅延生成器は、可変複屈折要素３０４を含む。適切な可変複屈折要素は、以下に限定されないが、可変リターダを含む。好適には、可変リターダは、電気光学の及び／又は液晶の波長板を含む。他の実施形態において、位相遅延生成器は、偏光成分を異なる長さの光路を介して伝播させる要素を含む。
【００６２】
図３Ｂを参照すると、一実施形態において、位相遅延生成器３５４は、偏光分割ビームスプリッタ３５５と可変遅延要素３５７を含む。次いで、サンプル光信号は、偏光ビームスプリッタ（「ＰＢＳ」）３５５によって２つのビームへ分割され、一方の偏光成分（例えば、Ｅ_ｘ）は透過し、他方（例えば、Ｅ_ｙ）は反射される。各アームにおける４分の１波長板及び反射器３５９は、偏光を回転させ、ビームスプリッタ後の２つのビームを再結合する。可変位相遅延は、２つの伝播光路間に可変リターダンスを生じさせる可変遅延要素３５７により、分離されたビーム（即ち、偏光成分）の一方に導入される。可変遅延要素の可能な実施例は、以下に限定されないが、固定軸液晶リターダ、可変リターダンス波長板、及び少なくとも１つの４分の１波長板の背後に配置された空間的可変ミラーを含む。
【００６３】
図４を参照すると、回転子４０２と位相遅延生成器４０４の好適な実施形態が示される。好適には、回転子４０２は、第１の切り換え可能半波長板４０３と第２の切り換え可能半波長板４０５を含み、位相遅延生成器４０４は、半波長板４０３と４０５の速波軸４１３、４１５に対して固定された方位でそろえられた速波軸を有する可変リターダを含む。一実施形態において、第２の半波長板４０５の速波軸の方位は、第１の半波長板４０３の速波軸に対してφ／２（４２５）だけ回転し、サンプル光信号の偏光軸の回転方位を、位相遅延生成器４０４の光軸に対してφだけ変化させる。次いで、その方位は、半波長板のリターダンスをゼロに変更することによりリセットされ得る。
【００６４】
図３Ａと図３Ｂを再び参照すると、サンプル光信号は、位相遅延生成器３０４、３５４から干渉計３０６へ送られる。干渉計３０６は、サンプル光信号の直交する偏光成分を、それらの間で干渉を生じることができる条件下で混合し、それにより干渉信号を生成する。好適には、干渉計は、４５°直線検光子を含む。しかしながら、理解されるように、光信号の偏光成分を、それらの間で干渉を生じる条件下で再結合できる任意の光学要素又は光学要素の構成は、本発明の偏光監視装置の干渉計として機能することができる。
【００６５】
本発明の種々の実施形態において、干渉計により生成された干渉信号は、検出器により測定される。種々の他の実施形態において、干渉信号は、スペクトル的に連続した波長サブ帯域へ分散され、異なる検出器要素が、光伝送システムの特定の波長チャネルに対応する干渉信号を受け取る。検出器要素は、検出器のアレイ、例えば２つ又はそれより多い物理的に独立した検出器、或いは集積化された検出器アレイからなることができる。適切な検出器アレイの例には、以下に限定されないが、電荷結合素子（「ＣＣＤ」）のアレイ、集積化されたフォトダイオードのアレイ、及び別個の検出器のアレイが含まれる。電気通信の用途において、InGaAsフォトダイオードが、アレイ及び別個の検出器用途に好適である。しかしながら、本明細書及び特許請求の範囲において使用される場合、理解されるように、用語「検出器のアレイ」と「検出器アレイ」は交換可能に使用され得る。即ち、要素が用語「検出器のアレイ」によって説明される場合、理解されるようにその用語は「検出器アレイ」も包含し、逆の場合も同様である。
【００６６】
図３Ａと図３Ｂを再び参照すると、一実施形態において、干渉計３０６により生成された干渉信号は、サンプル光信号をスペクトル的に連続した波長サブ帯域へとスペクトル的に分散する波長分波器３０８を通過する。結果として、分波器３０８は、各波長サブ帯域に干渉信号を提供する。一実施形態において、光信号（例えば、ＤＷＤＭ信号等）の各波長チャネルは、少なくとも２つの波長サブ帯域へとスペクトル分散することによって分割される。好適には、波長分波器は、各波長チャネルを５〜１５個の波長サブ帯域へ分割する。適切な波長分波器の例には、以下に限定されないが、自由空間及び平面の実施において、アレイ導波路格子（ＡＷＧ）、体積位相グレーティング（volume phase grating）分光計、及び反射グレーティング分光計が含まれる。
【００６７】
干渉信号を各波長サブ帯域の干渉信号へとスペクトル的に分散することにより、光信号の多重波長チャネルのＰＳＰ特性の測定が容易になる。更に、検出器アレイにわたって干渉信号をスペクトル的に分散させることにより、多重チャネルの干渉信号が実質的に同時に測定されることが可能になる。例えば、一実施形態において、波長分波器３０８は、検出器アレイ３１０にわたるスペクトル的に連続したサブ帯域へ干渉信号をスペクトル的に分散する。次いで、検出器アレイ３１０の素子の異なる集合が、異なる波長チャネルに対応する干渉信号を実質的に同時に受け取る。結果として、多重波長チャネルの干渉信号が実質的に同時に測定されることができ、この情報が偏光状態生成器によって使用され、多重波長チャネルに関するＰＳＰの相対的振幅と偏光状態及びＤＧＤが求められる。
【００６８】
偏光状態生成器は、アナログ装置及び／又はデジタル装置からなることができる。偏光状態生成器は、光信号のＰＳＰの偏光状態と相対的振幅、及びそれらの間のＤＧＤを求める。一実施形態において、偏光状態生成器は、サンプル光信号の２つまたはそれより多い回転方位のそれぞれにおける複数の位相遅延に関して測定された干渉信号に基づいてＰＳＰ特性を求める。更に、偏光状態生成器は、サンプル光信号の１つ又は複数の回転方位のそれぞれにおける複数の位相遅延に関して測定された干渉信号に基づいて多重波長チャネルのＰＳＰ特性を実質的に同時に求めることができる。好適には、偏光状態生成器は、異なる波長チャネル内の異なる波長サブ帯域について観測された干渉信号間の関係に基づいても多重波長チャネルのＰＳＰ特性を求める。
【００６９】
一実施形態において、偏光状態生成器は、実質的に式（３）〜（６）によるロジックを用いてＰＳＰ特性を求める。別の実施形態において、偏光状態生成器は、実質的に式（７）〜（１９）によるロジックを用いてＰＳＰ特性を求める。しかしながら、理解されるように、偏光状態生成器は、サンプル光信号の１つ又は複数の回転方位のそれぞれにおける複数の位相遅延の干渉信号に基づいてＰＳＰ特性を求めるのに適した任意のロジックを使用できる。
【００７０】
偏光状態生成器のロジックは、アナログ回路、デジタル回路によって、及び／又は汎用コンピュータのソフトウェアとして実施され得る。偏光状態生成器は、ＰＳＰの相対的振幅と偏光状態、及びそれらの間のＤＧＤの、例えばプリンタ又はコンピュータスクリーンによって生成されるような人間読取り可能な表示を生成することができる。しかしながら、偏光状態生成器が人間読取り可能な出力又は機械読取り可能な出力のみを生成するか否かは、本発明にとってあまり重要ではない。例えば、偏光状態生成器は、光伝送システムのＰＭＤを補償する偏光制御装置を制御するのに十分な機械読取り可能電気信号のみを生成してもよい。
【００７１】
理解されるように、本発明の偏光監視装置の様々な実施形態は、求められる偏光特性が任意の波長チャネルにおける光のストークスベクトルを計算するのに十分であるので、マルチチャネルストークスベクトル偏光計として使用することに適する。また、偏光監視装置の実施形態は、１つの波長チャネルのスペクトルによって照射される１つ又は複数の検出器素子上の平均強度が、チャネルの平均パワーの測定をもたらすので、マルチチャネルパワー監視装置として使用することにも適する。最後に、偏光監視装置の実施形態は、各監視装置の検出器チャネルの平均強度が、検出器サイズとグレーティングの分散に対応するスペクトルのビンにおけるパワーの測定をもたらすのでマルチチャネルスペクトルアナライザとして使用され得る。
【００７２】
一実施形態において、本発明は、光伝送システムにおいて光信号のＰＭＤを補償するための装置を提供する。図５と図６を参照すると、ＰＭＤ補償装置の種々の実施形態が示される。補償装置は、偏光監視装置５０３、６０３、偏光状態生成器５１７、６１７、及び補償段５２１、６２１を含む。偏光監視装置と補償段は、本明細書で説明される任意の実施形態を含むことができる。例示されるように、図５の偏光監視装置５０３は、図３Ａとそれに伴う説明で示されたものと実質的に同じであり、図６の偏光監視装置６０３は、図３Ｂとそれに伴う説明で示されたものと実質的に同じである。
【００７３】
動作において、偏光監視装置５０３、６０３は、サンプル光信号の１つ又は複数の回転方位のそれぞれに対する複数の位相遅延における干渉信号の測定値を偏光状態生成器５１７、６１７に提供する。次いで、偏光状態生成器５１７、６１７は、偏光監視装置５０３、６０３によって提供された測定値に基づいて、光信号、或いは光信号の１つ又は複数の波長チャネルのＰＳＰ特性を求める。一実施形態において、偏光状態生成器５１７、６１７は、補償段５２１、６２１の偏光制御装置５２２、６２２に制御信号を提供する偏光制御装置ドライバ５３０、６３０に制御信号を提供し、次いで補償段５２１、６２１は、光信号のエネルギーを光伝送システムの実質的に１つのＰＳＰへ移すような選択可能な方位において光信号にＤＧＤを追加する。代案として、前述したように、補償段は、１つのＰＳＰの偏光状態が光信号のものと実質的に同じであるように光伝送システムのＰＳＰを修正するものとも見なされ得る。
【００７４】
図５と図６を再び参照すると、種々の実施形態において、補償段は、偏光制御装置５２２、６２２、及び遅延要素５２４、６２４を含む。好適には、遅延要素５２４、６２４は、補償されるべきＰＭＤに関連した光伝送システムのＤＧＤより大きい実質的に固定されたＤＧＤを有する。従って、遅延要素５２４、６２４が実質的に固定されたＤＧＤを有する場合、偏光制御装置ドライバ５３０、６３０は、偏光制御装置５２２、６２２に制御信号のみを提供する。しかしながら、理解されるように、偏光制御装置ドライバ５３０、６３０及び／又は偏光状態生成器５１７、６１７は、センサ（例えば、温度、位置、パワー他）に限定されないが、システム診断及び制御システム、システムクロック他を含む種々のソースから入力信号を受け取り、ＰＭＤ補償を容易にすることができる。例えば、遅延要素５２４、６２４のＤＧＤは、温度と共に変動し、結果として、遅延要素温度センサからの入力信号を用いて遅延要素のＤＧＤを求めることができる。
【００７５】
図５と図６に例示されるように、補償段５２１、６２１は、光伝送システムの多重波長チャネルにおいてＰＭＤを補償するように適合される。例示された実施形態において、補償段５２１、６２１は、（ソースからレシーバまでの一般的な光信号の光路の順序で）波長分波器５２６、６２６、偏光制御装置５２２、６２２、波長合波器５２８、６２８、及び遅延要素５２４、６２４を含む。波長分波器５２６、６２６は、線５５１、６５１により表されるように、光信号をスペクトル的に連続した波長サブ帯域へ分散する。波長チャネルのエネルギーの十分な部分量が、対応する波長チャネルの単一のＰＳＰ内にあるような方位において、遅延要素５２４、６２４のＤＧＤが追加されるように、偏光制御装置は、各波長チャネルで動作する。概して、偏光制御装置は、各波長チャネルの異なる方位においてＤＧＤを追加する。その結果として、波長合波器は、偏光制御装置によって操作された光（線５５３、６５３によって表される）を再結合する。
【００７６】
図５と図６に例示されるように、補償段５２１、６２１は、点５０１、６０１から点５９９、６９９まで進む光信号の光路が光学監視装置に遭遇する前に補償段を通過するという意味において、偏光監視装置５０３、６０３に先行する。しかしながら、理解されるべきは、補償段の配置が偏光監視装置の「前」又は「後」であるということは本発明にとってあまり重要ではない。例えば、偏光監視装置が偏光制御装置に先行する場合、偏光制御装置のジョーンズ又はミュラー行列を用いて偏光制御装置の２つの端部間で偏光状態を変換できる。
【００７７】
本発明において有用な補償段は、種々の形態をとることができる。好適には、制御装置の遅延要素は、偏光保存ファイバ（「ＰＭＦ」）を含む。しかしながら、適切な遅延要素は、以下に限定されないが、自由空間及びファイバ遅延要素、或いは方解石又はバナジン酸イットリウムのような複屈折結晶を含む。適切な波長合波器及び分波器は、図３Ａ、図３Ｂ、図５、及び図６に例示されるように、グレーティング３０７、５３７、６３７、及び分散コリメータ３０９、５３９、６３９を含むことができる。更なる適切な波長分波器及び合波器は、以下に限定されないが、自由空間及び平面の実施において、アレイ導波路格子（ＡＷＧ）、体積位相グレーティング分光計、及び反射グレーティング分光計を含む。また、適切な偏光制御装置も、種々の形態をとることができる。例えば、偏光制御装置は、電気光学結晶から構成された一連の可変波長板、ニオブ酸リチウム波長板、液晶、ファイバスクィーザ、及びシリカ系応力リターダを含むことができる。
【００７８】
好適には、補償段は、実質的に固定されたＤＧＤを有する遅延要素とＬＣ偏光コントローラとを含む。図７を参照すると、ＬＣ偏光コントローラの好適な実施形態が示される。ＬＣ偏光コントローラ７０１は、４つのＬＣ波長板７０２、７０４、７０６、及び７０８のスタックを含む。好適には、ＬＣ波長板のスタックは、１０ｍｍ未満の厚みであり、好適にはＬＣ波長板の絶対的リターダンス誤差は、波長板当たり４ｎｍより大きくない。
【００７９】
好適には、各ＬＣ波長板は、導電性インジウムスズ酸化物（ＩＴＯ）のコーティングで被覆された光学品質ガラスの間に収容されたＬＣセル７１０のアレイを含む。それぞれの個々のＬＣセルは、対象となる波長帯域において実質的に０から１．２波までの可変波長板として機能し、個々のＩＴＯ電極により制御され得る。ＬＣセルは、好適には３ｍｓ未満の応答時間で電気的に制御される。例えば、素子の温度を上げることにより、一時的に応答を短くできる。
【００８０】
４つの波長板７０２、７０４、７０６、及び７０８は好適には、それぞれ０°、４５°、０°、及び４５°において公称上それらのこすり（rub：磨く）方向を有するように構成され、ガラスに対して対象となる波長領域に整合された屈折率の光学的に透明なエポキシで互いに積層される。好適には、セルの位置合わせ（ｘ−ｙ方向において）は、第１のＬＣ波長板７０２と最後のＬＣ波長板７０８との間で２５μｍ余りである。更に、ＬＣ偏光コントローラは、前部集積レンズアセンブリ７１２及び／又は後部集積レンズアセンブリ７１４を更に含むことができる。
【００８１】
好適には、個々のＬＣセルは、各通信チャネルの分波器が各セルの活性領域を介して伝送した後に各ＩＴＵ格子の間隔の９０％より大きいようなサイズになっている。言い換えれば、ＩＴＵチャネル間の帯域幅の１０％未満が、ＬＣセル間のセル間ギャップ７１６から失われるのが好ましい。
【００８２】
幾つかの実施形態において、上述した方法の機能は、汎用コンピュータのソフトウェアとして実施され得る。更に、係るプログラムは、コンピュータのランダムアクセスメモリの一部分に取っておき、回転子の制御、位相遅延生成器の制御、干渉計の制御、干渉信号の測定、偏光制御装置の制御、並びに測定された干渉信号との動作及びその干渉信号に対する操作に影響を与える制御ロジックを提供することができる。係る実施形態において、プログラムは、フォートラン、パスカル、Ｃ、Ｃ＋＋、又はベーシックのような多数の高レベル言語の任意の１つによって書かれ得る。更に、プログラムは、スクリプト、マクロ、或いはエクセル又はビジュアルベーシックのような市販ソフトウェアに組み込まれた機能で書かれ得る。更に、ソフトウェアは、コンピュータに常駐するマイクロプロセッサ向けのアセンブリ言語で実施され得る。例えば、ソフトウェアがＩＢＭ ＰＣ又はＰＣクローンで実行するように構成された場合、それはインテル８０ｘ８６アセンブリ言語で実施され得る。ソフトウェアは、以下に限定されないが、フロッピー（Ｒ）ディスク、ハードディスク、光ディスク、磁気テープ、ＰＲＯＭ、ＥＰＲＯＭ、又はＣＤ−ＲＯＭのような「コンピュータ読取り可能媒体」を含む製造品に組み込まれ得る。
【００８３】
偏光監視及びＰＭＤ補償の例
図５と図８を参照すると、光伝送システムにおいてＰＭＤに対する監視と補償を行う一実施形態の例は、以下の通りである。サンプル光信号は、９０／１０光学タップ５０２でもって光伝送システムから分岐され、光結合器５０４を介して偏光監視装置５０３へ入力する。偏光監視装置は、偏光回転子５０６、可変リターダからなる位相遅延生成器５０８、４５°直線偏光子からなる干渉計５１０、波長分波器５１２、及び多素子熱電子（「ＴＥ」）冷却されるInGaAsアレイからなる検出器のアレイ５１４を含む。
【００８４】
図５の偏光監視装置構成のこの例において、単一の更新期間に関するデータ収集の順序は、以下の通りである。即ち、
（１）サンプル光信号の偏光軸を角度φ_１（例えば、０°）だけ回転させるように偏光回転子５０６を設定する。
（２）可変リターダ５０８によって提供される位相遅延（例えば、ディザ位相遅延）を、例えば次のものを通じて変更する：
ａ．０〜１の波の正弦曲線的変化のような連続周期的リターダンス（位相遅延）特性、又は
ｂ．幾つかの別個のリターダンス（位相遅延）ステップ。
（３）ステップ２の間、各回転子−リターダ設定に対する波長分波器５１２によって提供されるスペクトル的に連続した波長帯域の干渉信号を検出器アレイ５１４で測定する。
（４）サンプル光信号の偏光軸を角度φ_２（例えば、４５°）だけ回転させるように偏光回転子５０６を設定する。
（５）ステップ２と３を繰り返す。
代案として、偏光回転子は、位相遅延がゆっくりと変更される又はステップされるように、２つの回転方位間のサンプル光信号をディザリングすることができる。
【００８５】
この例に関するデータ収集制御信号は、図８に模式的に示される。偏光回転子は、５０％のデューティサイクルで動作し、即ち実質的に等しい時間が、回転子制御信号パターン８０１によって示されるように２つの回転方位φ_１８１１とφ_２８１２で費やされる。各偏光回転子の位置、即ちサンプル光信号の各回転方位において、位相が位相遅延生成器でディザリングされ、位相ディザ時間ウィンドウ８３１内のサンプル光信号の偏光成分間に複数の位相遅延を生じる。位相ディザは、位相遅延又は一連の位相遅延ステップの連続した一時的変動とすることができる。位相遅延は、その全波又は一部分を通じて変更され得る。位相遅延生成器制御信号パターン８０３によって示されるように、この例において、位相ディザは、時間ウィンドウ８３１中に１の全波λと０の間に４つの位相遅延ステップを含む。
【００８６】
図５と図８に関連して、データ収集の間、位相遅延されたサンプル光信号は、干渉計５１０を通過し、波長分波器５１２でもって検出器アレイ５１４上へスペクトル的に分散される。検出器アレイは、波長チャネルに分散された信号を測定し、各チャネルはアレイの素子の異なる集合にある。従って、検出器アレイは、波長チャネルの全てに関して実質的に同時に干渉信号を測定する。検出器アレイのデータ取得パターン８０２は、偏光回転子と位相遅延生成器の変化量に対する干渉信号の検出器測定のタイミング（即ち、検出器の露光）８２１を示す。位相遅延は、各検出器の露光にわたって一定に保持されるか、又は傾斜の変化をつけられてもよい。
【００８７】
偏光状態生成器５１７は、光信号の所望のスペクトル幅に広がる波長サブ帯域の全ての干渉信号を処理する。偏光状態生成器の処理のパターン８０４は、他のデータ収集動作に対する干渉信号測定処理のタイミング８４１を示す。偏光状態生成器は、ＰＳＰ特性を求め、この情報に基づいて、偏光制御ベクトルが、光信号の所望のスペクトル幅に広がる波長チャネルについて求められる。制御ベクトルの測定のパターン８０６は、他のデータ収集と処理動作に対する偏光制御ベクトルの測定のタイミング８６１を示す。この例において、偏光制御ベクトルは、実質的に式（７）〜（１２）、及び（１３）〜（１９）に従って各波長チャネルの干渉信号の測定された強度から求められる。
【００８８】
偏光制御ベクトルに基づいて、偏光制御装置ドライバ５３０が、偏光制御装置５２２の駆動信号を決定し、偏光制御装置５２２は、各波長チャネルの光信号エネルギーの十分な部分量をチャネルの単一のＰＳＰへと移す。偏光制御装置ドライバのパターン８０８は、他のデータ収集と処理動作に対する偏光制御装置の駆動信号の印加のタイミング８８１を示す。
【００８９】
好適な実施形態において、補償段は、以下の通りに光信号のＰＭＤを補償する。最初に、信号が波長分波器５２６によってスペクトル的に分散され、１つの波長チャネルがマルチチャネル偏光制御装置５２２の各チャネル（即ち、素子の集合）を通過する。次いで、その波長チャネルの光が、波長合波器５２８によって再結合され、補償されるべきＤＧＤの量より大きいＤＧＤを有する単一の偏光保存ファイバ５２４からなる遅延要素へ送らる。偏光制御装置５２２は、光信号の偏光状態が伝送システムにＰＭＤ補償器を加えた組み合わせのＰＳＰに一致するように各チャネルの偏光状態を変更する。好適には、偏光監視装置５０３のサンプル光信号は、偏光制御装置のフィードバック信号と診断信号が求められ得るように偏光制御装置の後で伝送システムから分岐される。しかしながら、理解されるべきことは、上述したように偏光制御装置のフィードバック及び／又は反復の制御は、本発明にとってあまり重要ではない。
【００９０】
図８に示されるように、データ収集の開始からＰＭＤ補償の完了までの時間間隔、即ちこの例における更新サイクル８０７は、８ｍｓである。時間のパターン８０５は、この時間の約３ｍｓがデータ収集に費やされ、そのデータ収集において約２ｍｓが複数の位相遅延及び第１の検出器露光８５１における回転方位において干渉信号を生成することに費やされ、約１ｍｓが、複数の位相遅延及び第２の検出器露光８５２における回転方位において干渉信号を生成することに費やされる。更に、一実施形態において、各検出器露光期間８５１、８５２に関して、少なくとも６回の測定８５５、８５６が行われる。例示されたように、第１の検出器露光期間８５１に関して、測定８５５は実質的に等しい持続時間からなる（例えば、各測定は、第１の検出器露光期間８５１及び６回の測定８５５の２ｍｓに関して１ｍｓの長さの約１／３である）。同様に、第２の検出器露光期間８５２について、測定８５６は実質的に等しい持続時間からなる（例えば、各測定は、第２の検出器露光期間８５２及び６回の測定８５６の１ｍｓに関して１ｍｓの長さの約１／６である）。
【００９１】
時間のパターン８０５は、この例における更新サイクルの約２ｍｓがデータの処理及び／又は偏光回転子と位相遅延生成器の傾斜付け８５３に費やされ、約３ｍｓが偏光制御装置５２２の駆動及び光信号のＰＭＤ補償に費やされることを更に示す。特定の用途において、データ収集とＰＭＤ補償のプロセスは、ＰＭＤの度合いの変化及び／又はある期間にわたる伝送システムのＰＳＰの変化に対処するために繰り返される。この例において、本発明によって提供されるＰＭＤ補償サイクルは、１２５Ｈｚの周波数で動作する。
【００９２】
本発明は、特定の実施形態に関連して特に図示されて説明されてきたが、当業者によって理解されるように、添付の特許請求の範囲によって規定されるような本発明の思想と範囲から逸脱することなく、形態及び細部に様々な変更を行うことができる。従って、本発明の範囲は、添付の特許請求の範囲によって示され、それ故に、特許請求の範囲の等価物の意味と範囲内に入る全ての変更は、包含されることが意図されている。
【図面の簡単な説明】
【００９３】
【図１】光伝送システムのＰＭＤベクトルとＰＳＰのポアンカレ球による表現を示す図である。
【図２Ａ】本発明のＰＭＤ補償方法の一実施形態を示す図である。
【図２Ｂ】本発明のＰＭＤ補償方法の一実施形態を示す図である。
【図３Ａ】本発明の偏光監視装置の種々の実施形態を示す略図である。
【図３Ｂ】本発明の偏光監視装置の種々の実施形態を示す略図である。
【図４】本発明の回転子と位相遅延装置の一実施形態を示す略図である。
【図５】本発明のＰＭＤ補償装置の一実施形態を示す略図である。
【図６】本発明のＰＭＤ補償装置の一実施形態を示す略図である。
【図７】本発明の補償段の偏光コントローラの一実施形態を示す略図である。
【図８】本発明の偏光監視方法の一実施形態を示す略図である。【Technical field】
[0001]
The present invention relates to the field of optical transmission systems. In particular, the invention relates to monitoring and modifying optical signals in optical transmission media.
[0002]
Cross reference to related application
This invention claims the benefit of US Provisional Application Serial No. 60 / 276,982, filed March 19, 2001.
[Background Art]
[0003]
An input data stream of an optical transmission system can be viewed as a series of write pulses representing digital bits. The bit rates of current optical transmission systems range from 10 GHz to 40 GHz, typically resulting in write pulses (or bit periods) each being 100 to 25 picoseconds wide. The receiver of the optical transmission system determines, for each bit period, whether a write pulse has been received (digital 1) or not (digital 0) by each bit period of the data stream. To a digital one or zero. Polarization mode dispersion (PMD) is a phenomenon that can distort the write pulses of a data stream and thus reduce the ability of a receiver to determine whether to convert a bit period to a one or a zero. As a result, PMD limits the transmission accuracy and capabilities of optical transmission systems.
[0004]
Polarization mode dispersion results from the birefringence of the transmission medium of an optical transmission system. Due to the imperfections and asymmetric stresses of the optical fiber that result in a non-circular fiber core, birefringence exists even in the transmission medium consisting of what is called "single-mode" optical fiber. An ideal single mode optical fiber has a circular core, ie, a core that is isotropic and non-eccentric. Such an ideal fiber is isotropic, that is, the index of refraction of the fiber is independent of the electric field, or in other words, the direction of polarization. The anisotropy (e.g., eccentricity) of an optical fiber core leads to birefringence, so that different polarizations propagate through the optical fiber at different velocities.
[0005]
The propagation of light in an optical fiber can be considered to be controlled by two fundamental modes or major modes. These principal modes are known as "primary polarization states (PSP)". If a PSP is introduced into the fiber link, the polarization at the output of the link will be substantially constant for the first order of frequency. In an ideal single mode fiber, the PSP is degenerate, or indistinguishable. The anisotropy of the fiber core overcomes this degeneration. As a result, the PSP travels at different group velocities and separates into two temporally displaced pulses. The separation of PSPs due to different group velocities is known as polarization mode dispersion (PMD), and the temporal spread between two PSPs is known as group delay time difference. Due to this temporal spread, a write pulse in one bit period of the data stream may overlap another bit period. This overlap reduces the ability of the receiver to determine whether to convert a bit period to a one or a zero. Therefore, PMD is a problem for optical transmission systems, resulting in data ambiguity, data loss, data contamination, and limited transmission capabilities.
[0006]
Various methods have been proposed for the PMD problem, each of which has its limitations. For example, polarization-maintaining optical fibers maintain the polarization of the input through unique optical properties such as stress-induced anisotropy introduced by internal stress members into the birefringent fiber, and reduce the optical power between the PSPs. Designed to prevent cross coupling. Unfortunately, this specialty fiber is not only expensive, does not extend to large-scale replacement, and cannot address PMD in existing "heritage" fiber networks.
[0007]
Current electronic methods, such as electrical distortion equalizers, also exhibit disadvantages. These methods, which generally use the notch of the RF frequency response (ie, the minimum value of the response) in the receiver as an indicator of DGD, require modification of conventional receiver electronics and require high speed digital or RF electronics. Tend to need.
[0008]
Optical measurement methods generally require that the laser source be perturbed by polarization scrambling or by the introduction of frequency sidebands, or that only indirect or qualitative measurements of the polarization properties of the PMD be made. And In optical transmission systems, perturbing the laser source for optical measurements is generally not practical and interrupts data transmission. Methods that utilize only indirect or qualitative measurements of PMD polarization properties, such as DGD and Degree of Polarization ("DoP") measurements, require the use of an iterative procedure that compensates for PMD only after multiple operations. I do. However, such operations are time consuming, and thus such repetitive compensation methods have drawbacks with respect to applications for high speed transmission systems.
[0009]
Therefore, there is a need for a method of providing a reliable PMD measurement that allows faster compensation for the effects of PMD without interrupting data transmission.
[0010]
Summary of the Invention
The present invention provides a method and apparatus for obtaining a direct measurement of PMD polarization properties without perturbing the laser source and compensating for the effects of PMD in a single operation. The present invention compensates for PMD by substantially moving the optical signal of an optical transmission system to a single PSP in a system that includes a compensator. As a result, each write pulse of the data stream is not substantially mixed with a temporally adjacent write pulse or bit period.
[0011]
In one aspect, the present invention provides a method for compensating for PMD of an optical signal in an optical transmission system. In one embodiment, the method measures the PMD polarization characteristics of a fiber link by introducing a phase delay between two different polarization components of the sample optical signal from the optical transmission medium. The method causes interference between the two polarization components and measures the resulting interference signal. The method then determines the relative amplitude and polarization state (eg, orientation and ellipticity) of the DGD and PSP using the interference signals measured at the multiple phase delays. The relative amplitude and polarization state of the DGD and PSP provide a direct measure of PMD polarization properties. As used herein, the term “PSP characteristic” refers to the polarization state and relative amplitude of the PSP, and the DGD between them. The polarization state and the relative amplitude of the PSP also provide a direct measure of the polarization state of an optical signal in an optical transmission system. Based on the PSP characteristics, the present invention provides a method for moving an optical signal that transfers a sufficient fraction of the optical signal energy to a single PSP of an optical transmission system, preferably in a single operation, to compensate for PMD effects. Call for correction. The "sufficient sub-quantity" may be selected by those skilled in the art, for example, to provide an appropriate system outage probability or power penalty.
[0012]
As used herein, the term "sufficient partial amount" refers to an amount that is sufficient to prevent a bit error rate due to PMD effects for a particular transmission system or data transmission. For example, if the data transmission involves data that is very redundant and resistant to corruption, the sufficient fraction can be reduced. Similarly, if only a low data transmission rate is desired, the sufficient fractional amount can be reduced. Conversely, if it is desired to operate the transmission system with high capacity and / or high data accuracy (ie, low ambiguity, low loss, or low corruption bit error rate of the data), a sufficient fractional amount is very small. Or even include substantially transferring the entire energy of the optical signal to a single PSP. Thus, as will be appreciated by those skilled in the art, a sufficient fraction of the optical signal energy to transfer can be determined in a simple manner (without undue experimentation). For example, a sufficient portion may include substantially all of the optical signal energy.
[0013]
In another embodiment, the method measures PMD polarization properties by rotating the orientation of the polarization axis of the sample optical signal and introducing a phase delay between two different polarization components of the sample optical signal. The method causes interference between the two polarization components and measures the resulting interference signal. The method then uses the interference signal measured with two or more phase delays for each of two or more rotational orientations of the polarization axis of the sample optical signal to determine the PSP characteristics of the optical signal. Ask. Based on PSP characteristics, the present invention seeks a modification to an optical signal that moves a sufficient fraction of the optical signal energy to a single PSP in an optical transmission system in a single operation.
[0014]
In another embodiment, the method of the present invention provides that a single operation transfers a sufficient fraction of the optical signal energy to a single PSP of an optical transmission system embodying an incident fiber link and a PMD compensator. In such an operation, the PMD of the optical transmission system is compensated by adding a DGD vector to the incident optical signal.
[0015]
In a preferred embodiment, the present invention determines the PSP characteristics of multiple wavelength channels in an optical fiber, substantially in parallel, as present in dense wavelength division multiplexed ("DWDM") fibers. These properties include the polarization state of the PSP, the relative amplitude of the PSP (ie, the energy ratio between the two PSPs), the DGD, and the sum of the power for the multiple wavelength channels of the data stream. The bandwidth of each wavelength channel is determined mainly by the line width of the laser source and the data modulation bandwidth of the optical signal.
[0016]
In one version of this embodiment, the method introduces a phase delay between two different polarization components of the sample optical signal. The method causes two polarization components to interfere to generate an interference signal, disperses the interference signal into spectrally continuous wavelength bands, and measures the interference signal for each wavelength sub-band. Then, the method measures an interference signal for each wavelength sub-band in the plurality of phase delays and determines a PSP characteristic for each wavelength channel. Based on the PSP characteristics of a channel, the present invention determines a modification to the optical signal of each wavelength channel that transfers a sufficient portion of the optical signal energy of that wavelength channel to a single PSP for that channel in a single operation. I do.
[0017]
In another version of this embodiment, the method rotates the orientation of the polarization axis of the sample optical signal and introduces a phase delay between two different polarization components of the sample optical signal. The method causes interference between two polarization components to generate an interference signal, disperses the interference signal of each channel into spectrally continuous wavelength sub-bands, and measures the interference signal for each wavelength sub-band. The method then measures the interference signal of each wavelength sub-band at two or more phase delays for each of the two or more rotation orientations of the optical signal polarization axis and calculates the PSP characteristics of the corresponding wavelength channel. Ask for. Based on the PSP characteristics of the channel, the present invention determines a modification to the optical signal that moves a sufficient portion of the optical signal energy of the wavelength channel to a single PSP of the channel in a single operation.
[0018]
In a preferred embodiment, the method of the present invention compensates for PMD of two or more wavelength channels of an optical transmission system. The method substantially adds DGD to each channel at substantially the same time in a direction such that a single operation transfers a sufficient portion of the optical signal energy of the channel to a single PSP of the channel. At the same time, the PMD of the wavelength channel is compensated.
[0019]
In another embodiment, the optical signal is sampled and analyzed, and modifications to the optical signal can be intermittent, periodic, or continuous, as the degree of PMD and the polarization state of the PSP may change over time. Will be updated.
[0020]
In another aspect, the present invention relates to a method, wherein the functionality of the method of the present invention includes, but is not limited to, a floppy disk, hard disk, optical disk, magnetic tape, PROM, EPROM, CD-ROM, or DVD-ROM. An article of manufacture embodied in such a computer-readable medium is provided.
[0021]
In another aspect, the present invention provides an apparatus for compensating PMD of an optical signal of an optical transmission system. In one embodiment, the device includes an optical polarization monitor and a polarization state generator. The polarization monitor includes a phase delay generator, an interferometer, and a detector. The polarization monitor is configured to accept a sample optical signal, and a phase delay generator introduces a phase delay between two different polarization components of the sample optical signal. An interferometer is arranged to receive the phase-delayed light and interfere with the two polarization components to generate an interference signal measured by the detector. The polarization state generator determines the relative amplitude and polarization state of the DGD and PSP based on the interference signals measured at the multiple phase delays.
[0022]
In another preferred embodiment, the polarization monitoring device also includes a rotator. The polarization monitor is configured to accept a sample optical signal. The rotator provides at least two rotational orientations of the polarization axis of the sample optical signal with respect to the optical axis of the phase delay generator. The phase delay generator introduces a phase delay between two different polarization components of the sample optical signal for each of the rotation orientations of the polarization axis. The interferometer is arranged to receive the phase-delayed light, cause interference between the two polarization components, and generate an interference signal measured by the detector. The polarization state generator determines the relative amplitude and polarization state of the DGD and PSP based on the interference signals measured at two or more phase delays for each of the two or more rotation orientations. Ask.
[0023]
In one embodiment, the rotator includes an electro-optic element that effectively rotates the optical axis of the phase delay generator. In another embodiment, the rotator includes a mechanism to physically rotate the phase delay generator. Preferably, the rotator comprises a polarization rotator for rotating the polarization axis of the sample optical signal. Suitable polarization rotators include, but are not limited to, a combination of a Faraday rotator and a wave plate.
[0024]
In another embodiment, the polarization monitor determines PSP characteristics of two or more wavelength channels of the optical transmission system substantially simultaneously. In one version of this embodiment, the polarization monitor includes a phase delay generator, an interferometer, a wavelength demultiplexer, and an array of detectors. The polarization monitoring device also includes a rotator. The interferometer is arranged to receive the phase delayed light and cause interference between the two polarization components to generate an interference signal. The splitter disperses the interfering signal into spectrally continuous wavelength subbands of the array of detectors. The array of detectors is configured such that the interference signals of each wavelength sub-band are measured substantially simultaneously. The polarization state generator then determines, for each wavelength channel, a corresponding wavelength sub-band interference signal measured at a plurality of phase delays or at two or more rotational orientations of the sample optical signal's polarization axis. Find the PSP characteristics.
[0025]
In another embodiment, the present invention provides an apparatus for compensating PMD of an optical signal of an optical transmission system, the apparatus comprising an optical polarization monitor, a polarization state generator, a polarization controller, and a delay element. including. The polarization controller modifies the optical signal such that a sufficient fraction of the energy of the optical signal is transferred to a single PSP in the optical transmission system. In one version of this embodiment, the compensation stage includes a polarization controller that changes the polarization state incident on the delay element. The delay element then adds a substantially selectable DGD in the selectable orientation to the optical signal. In a preferred version of this embodiment, the compensation stage includes a polarization controller and a delay element that adds a substantially fixed DGD in the selectable orientation to the optical signal. The selected orientation is determined based on the PSP characteristics provided by the polarization state generator. The selected orientation is such that when the compensation stage adds DGD to the optical signal, the resulting optical signal, ie, the modified optical signal, has a sufficient fraction of its energy in a single PSP of the optical transmission system. It has become.
[0026]
In another embodiment, the present invention provides an apparatus for compensating for PMD of two or more wavelength channels of an optical transmission system. The apparatus includes a polarization monitor for monitoring the PSP characteristics of the wavelength channel, a polarization state generator, and a multi-channel polarization controller. The multi-channel polarization controller modifies the optical signal of each wavelength channel such that a sufficient fraction of the optical energy in each channel is transferred to a single PSP in the channel. The multi-channel polarization control device includes a wavelength demultiplexer, a multi-channel polarization controller, and a wavelength multiplexer. The demultiplexer disperses the optical signal into the desired spectrally continuous channels, and the multi-channel polarization controller determines that a sufficient fraction of the energy of the wavelength channel is within a single PSP of the corresponding wavelength channel. In the azimuth, add DGD to each wavelength channel. As a result, the wavelength multiplexer recombines the light received from the multi-channel polarization controller. In one embodiment, the multi-channel polarization controller includes an array of polarization controllers, each polarization controller operating on a separate wavelength channel to introduce DGD in a selectable orientation when combined with a delay element. Preferably, the polarization controllers form a substantially integrated array and operate substantially simultaneously on the wavelength channels.
[0027]
In one version of this embodiment, the polarization controller adds substantially selectable DGD at selectable orientations to the optical signal of the wavelength channel. Preferably, the polarization controller adds a substantially fixed amount of DGD in a selectable orientation. The direction of the DGD added to the wavelength channel is selected based on the PSP characteristics of the channel. The selected orientation is such that as the variable polarizer adds DGD to the channel's optical signal, the resulting (ie, modified) optical signal will have a sufficient fraction of its energy in a single PSP for that wavelength channel. To have a quantity.
[0028]
The foregoing and other features and advantages of the invention, as well as the invention itself, will be more fully understood from the detailed description, drawings, and claims.
[0029]
Detailed description
The present invention provides a method and apparatus for monitoring and modifying optical signals that are particularly useful for optical transmission of data in fiber optic transmission systems. The present invention provides a polarization characteristic of light and / or a PSP for an optical transmission system. The polarization properties provided by the present invention are used to provide a "deterministic solution" to the PMD problem, substantially compensating the PMD of the optical signal for the first order. As used herein, the term “deterministic solution” refers to a single term for an optical signal that can substantially compensate for any PMD (ie, PMD at any one time) to the first order. Refers to the ability of the present invention to seek a modification of The deterministic solution method of the present invention contrasts with techniques that require repeated measurements and corrections to compensate for any PMD.
[0030]
One method of the present invention for PMD compensation is to transfer a sufficient fraction of the optical signal energy to a single PSP in an optical transmission system. A difficult problem exists in determining to quickly modify an optical signal to compensate for the system's PSP and PMD in a reliable manner without interrupting data transmission. As will be appreciated, the transfer of light to a single PSP can be accomplished in two ways: the optical signal may have substantially the same polarization vector orientation (eg, Stokes vector orientation) as that of the system PSP. Modifying the polarization state of the PMD vector of the system to have substantially the same PMD vector orientation as that of the incident optical signal.
[0031]
One intuitive aspect for understanding the PMD compensation method of the present invention involves the optical signal and Poincare sphere representation of an optical transmission system. The electric field vector E of an optical signal, such as in an optical fiber, generally has x and y components, ie, E_xWith constant phase offset ε_yAnd can be expressed as the sum of
[0032]
(Equation 1)

[0033]
The electric field vector is generally elliptically polarized, ie, E_xAnd E_yAre both nonzero and E over a period of time_xAnd E_yFollow the elliptical path of Linear and circular polarization degenerate the state of elliptically polarized light because the electric field vector draws a straight line or a circle, respectively, rather than an ellipse in time. One convenient way to represent all possible polarizations is with the Poincare sphere.
[0034]
Referring to FIG. 1, a representation 100 of a Poincare sphere of polarization states is shown. An arbitrary latitude on the spherical surface represents an arbitrary elliptically polarized light, a linearly polarized light is represented at the equator 101, and a circularly polarized light is represented at the pole 103. In this representation, one degree of longitude on the sphere represents a 0.5 degree physical rotation of the polarization axis. The left and right images of polarized light change in the two hemispheres, clockwise polarization in the upper hemisphere, and counterclockwise polarization in the lower hemisphere. Further, each longitude represents a fixed azimuth with respect to the semi-long axis of the polarization ellipse.
[0035]
In this representation, the PMD of the optical transmission system can be represented by a PMD vector (vector Ω) 102 on the Poincare sphere. The direction of PMD vector (vector Ω) 102 represents one of the PSPs in the system, but the magnitude of the vector is half the DGD. The polarization state of any sample optical signal (ie, the input polarization state to the polarization monitor) is a linear combination of the two PSPs. The input polarization vector 104 (eg, Stokes vector, etc.) can be used to represent the optical signal polarization state. The relative amount of optical signal energy in one PSP is cos²(2θ), the other is sin²Given by (2θ), where (2θ) is the angle on the Poincare sphere between the input polarization vector 104 and the PMD vector (vector Ω) 102. If the input polarization vector 104 falls along the PMD vector 102, the total optical signal energy for the first order is within one PSP of the transmission system. Correspondingly, the energy of the optical signal for the first order is not present in other PSPs. As a result, one PSP in one bit period (eg, a fast PSP) may overlap with another PSP in another bit period (eg, a slow PSP), resulting in data loss, ambiguity, or contamination. ) Has no energy. The potential overlap of different PSPs at different bit periods due to PMD is referred to herein as PMD spreading.
[0036]
The components of the optical transmission system (eg, optical fiber sections, optical elements, couplers, multiplexers, switches, routers, etc.) can also be represented using PMD vectors, and the sum of these vectors in a common coordinate system. Is the total PMD vector of the system. As will be appreciated, the overall PMD vector of the system generally varies along the length of the transmission system and varies over time. For example, the total PMD vector 1 km from the optical signal source is generally different from that 10 km from the signal source. Similarly, the total PMD vector at any one location may change over time due to, for example, changes in temperature and / or stress in components of the system. Thus, as will be appreciated, the total PMD vector or system PMD vector refers to the PMD vector at a particular system location and time.
[0037]
Referring to FIG. 2A, a schematic vector diagram of one embodiment of compensating for PMD according to the present invention is shown. The method of the present invention is to transfer a sufficient fraction of the optical signal energy to a single PSP in an optical transmission system. The PMD vector of the optical transmission system having no contribution from the polarization controller and the delay element of the present invention is a vector (vector Ω)._LINK) 202. The circular portion 203 is a vector (vector Ω_LINK) 202 and the portion of the Poincare sphere in the plane defined by the input polarization vector 204. In one embodiment, the method of the present invention employs the resulting PMD vector (vector Ω_TOTAL) 208 has substantially the same alignment as input polarization vector 204 such that polarization control vector (vector Ω_PC) 206 to a vector (vector Ω_LINK) 202 can be represented. Here, the input polarization vector 204 is the PMD vector (vector Ω) of the new system._TOTAL) 208 substantially. As a result, a sufficient fraction of the optical signal energy travels to one PSP of the system, thereby compensating for the effects of PMD by substantially compensating for PMD spread to the first order.
[0038]
Referring to FIG. 2B, the effect of transferring a sufficient fraction of light energy to a single PSP is shown. The plot on the left 250 shows the optical signal energy of each PSP 252, 254 before PMD compensation, and the plot on the right 251 shows the optical signal energy of each PSP after PMD compensation according to the present invention. The area between the waveform representing the PSPs 252, 254 and the "zero" line 253 is proportional to the energy of the PSP represented by the waveform. In the embodiment of FIG. 2B, the PMD compensation provided by the present invention transfers a sufficient fraction of the optical signal energy from the fast PSP 254 to the slow PSP 252. However, as will be appreciated, transferring all of the optical signal energy to a single PSP is not critical to the present invention. Rather, a fraction of the energy that is insufficient to cause significant PMD diffusion can remain in the PSP as shown in plot 251 on the right due to the energy remaining in the fast PSP 254 after PMD compensation.
[0039]
As can be seen, the new system DGD, DGD_TOTALIs generally non-zero. For example, the length of the vectors in FIG. 2A is proportional to the DGD associated with each vector. Thus, for the embodiment shown in FIG. 2A, the DGD of the resulting system is:
[0040]
(Equation 2)

[0041]
Where DGD_LINKIs a vector (vector Ω_LINKDGD related to 202), and DGD_COMPIs the compensation vector (vector Ω_PC) 206 is the DGD. In one embodiment, the DGD_COMPIs greater than one bit period of the data stream, preferably DGD_COMPIs the PMD vector of the system substantially on the input polarization vector 204 (vector Ω_TOTALDGD to facilitate generation of 208)_LINKGreater than.
[0042]
In one aspect of the invention, the physical process represented by the addition of the polarization control vector is provided by a polarization controller and a delay element. The polarization controller and the delay element can provide a single polarization control vector or separate polarization control vectors for each of two or more wavelength channels. In one embodiment, the polarization controller includes a polarization controller and a delay element. The delay elements are DGD, DGD_COMPAnd the polarization controller provides a DGD for the optical signal on the Poincare sphere._COMPChange the direction of The delay element may provide a variable DGD, or preferably provide a substantially fixed DGD. The polarization controller and the delay element can consist of a single integrated optical component, or two or more optical components. For example, in one embodiment, the polarization controller consists of two optical components: a variable liquid crystal ("LC") polarization controller and a polarization preserving fiber, the variable LC polarization controller acting as a polarization controller, and the PMF as a delay element. work.
[0043]
The orientation selected for the polarization control vector is determined based on the PSP characteristics of the system. The polarization characteristics are determined from a sample optical signal that includes at least a portion of the optical signal of the optical transmission system. In one embodiment, the present invention introduces a phase delay between two different (preferably, but not necessarily, orthogonal) polarization components of the sampled optical signal so as to generate two interfering signals. Causes polarization components to interfere. Since each polarization component is generally a superposition of the two PSPs of the system, the resulting interference signal will include an auto-correlation component and a cross-correlation component between the PSPs.
[0044]
The polarization monitoring and PMD compensation method of the present invention is suitable for being applied substantially simultaneously to two or more wavelength channels of an optical transmission system. In one embodiment, the present invention introduces a phase delay between two different (preferably, but not necessarily, orthogonal) polarization components of the sampled optical signal so as to generate two interfering signals. Causes polarization components to interfere. The interfering signals are dispersed into spectrally continuous wavelength sub-bands to facilitate obtaining an interfering signal for each wavelength channel.
[0045]
The interfering signals obtained at multiple phase delays can be used to determine the relative amplitude and polarization state of the PSP and the DGD between them. Since the phase delay between the two polarization components varies (eg, through the full wave of delay), the intensity of the interfering signal varies sinusoidally over a portion of the period, and the observed portion of the period depends on the optical transmission system. DGD, ie DGD_LINKDepends on. As a result, the intensity I of the interference signal is₀Can be expressed as a sinusoidal function of the phase delay d in unit time:
I = I₀+ Ccos (dω₀) + Ssin (dω₀Equation (3)
The sinusoidal signal of equation (3) has a coefficient I₀, C, and S. From these coefficients, the polarization characteristics of the sample optical signal and the vector (vector Ω_LINK).
[0046]
Correspondingly, in embodiments where the interfering signal is obtained for two or more wavelength sub-bands, the intensity I (ω) of the interfering signal for each wavelength sub-band is calculated as a sinusoidal function of the phase delay as Can be represented as:
I (ω) = I₀+ Ccos (dω) + Ssin (dω) Equation (4)
Where ω is the frequency corresponding to the relevant wavelength sub-band. The sinusoidal signal of equation (4) is expressed by a coefficient I₀, C, and S. Furthermore, in the first order PMD limit, the coefficients of C and S are also sinusoidal functions of frequency and can be expressed as:
C (ω) = C₀+ C_ccos (τω) + C_ssin (τω) Equation (5)
S (ω) = S₀+ S_ccos (τω) + S_ssin (τω) Equation (6)
Here, τ is a time delay related to the DGD of the optical signal of the wavelength channel. For any data set, these equations can be solved for DGD τ and the coefficients of S (ω) and C (ω). From these coefficients, the PSP characteristics of each wavelength channel can be obtained.
[0047]
In another embodiment, the invention changes the rotation of the polarization axis with respect to the polarization component of the sample optical signal before introducing a second series of phase delays. This redundancy eliminates the case where the device cannot calculate all the required polarization information from the acquired data. The resulting interference signals obtained in two or more orientations provide additional information for the PSP's autocorrelation and cross-correlation. In one embodiment, the present invention introduces a phase delay between two polarization components of a sample optical signal for each of two or more rotation orientations, and then (2) for any rotation orientation. The two polarization components interfere to generate an interference signal. In one version of this embodiment, the interference signals are obtained for two different rotation orientations. In one version, the first rotation orientation is caused by a 0 ° rotation (ie, no rotation) of the polarization axis, and the second rotation orientation is caused by a 45 ° rotation of the polarization axis. However, as will be appreciated, the exact rotational orientation of the axis is not critical to the invention, but rather any set of non-degenerate rotational orientations may be used. The resulting interference signal for each rotational orientation can also be represented as a sinusoidal function of phase delay having the same functional form as equation (3).
[0048]
Further, the method of changing the rotation orientation of the polarization axis of the sample optical signal before introducing the phase delay is suitable for applying to two or more wavelength channels substantially simultaneously. In one embodiment, for each of the two or more rotation orientations, the present invention introduces a phase delay between the two polarization components of the sample optical signal to generate an interference signal for any rotation orientation. Causes interference between the two polarization components. The interfering signal is then dispersed into spectrally continuous wavelength sub-bands to facilitate obtaining an interfering signal for each wavelength channel for any rotational orientation. The resulting interference signal for each rotational orientation φ can also be represented as a sinusoidal function of phase delay having the same functional form as equation (4).
[0049]
For example, if two rotational orientations are used, such as φ = 0 ° and φ = 45 °, the resulting interference signal for each wavelength subband is expressed as:
[0050]
(Equation 3)

[0051]
These sinusoidal signals are, for each wavelength sub-band, I₀ ⁰, C⁰, I₀ ⁴⁵, C⁴⁵, And S⁴⁵Can be solved for Further, in the limit of the first-order PMD, C^φAnd S^φIs also a sinusoidal transverse frequency and can be expressed as:
[0052]
(Equation 4)

[0053]
For any data set, these equations can be solved for DGDτ and the coefficients of S (ω) and C (ω). From these coefficients, the PSP characteristics of each wavelength channel can be obtained.
[0054]
For example, the PSP characteristic of an optical signal of a wavelength channel can be obtained from the following equation.
[0055]
(Equation 5)

[0056]
Where I_inputRepresents the incident intensity on the monitoring device and is determined, for example, by summing the detected intensities for two dither settings separated by a half-wave, or by summing the intensity of a series of pairs of measurements. obtain. The PMD vector in the monitoring device is E_lx, E_ly, And .epsilon., While the vector length is determined by DGD.tau.
[0057]
In another aspect, the invention provides an apparatus that facilitates measuring a PSP property. In various embodiments, the apparatus includes a polarization monitor and a polarization state generator. Various embodiments of the polarization monitoring device according to the invention are described below in connection with a further description of the various embodiments relating to the method of the invention.
[0058]
Referring to FIGS. 3A and 3B, in various embodiments, a polarization monitoring device according to the present invention includes a rotator 302, phase delay generators 304, 305, an interferometer 306, a wavelength demultiplexer 308, and a detector 310. Including. As shown in FIGS. 3A and 3B, in one embodiment, the sample optical signal 301 passes through a rotator 302, which causes the rotation orientation of the polarization ellipse of the sample optical signal to be a phase delay generator 304, 354 optical axes. A suitable rotator is any element that can rotate the polarization axis of the optical signal (the polarization rotator) and / or rotate the optical axis of the phase delay generator, while substantially maintaining the ellipse of incidence. including,
[0059]
For example, suitable rotors include, but are not limited to, a Faraday rotator, two switchable half-wave plates whose fast-wave axes are aligned or separated by half the desired rotation angle in azimuth, and the fast-wave axis is Includes two variable phase retarders separated by half the desired rotation angle. In another embodiment, the rotator rotates the optical axis of the phase delay generator. Such a phase delay generator can include, for example, a phase delay generator, or an element that physically rotates a phase delay generator having a switchable optical axis.
[0060]
In other embodiments, the polarization monitoring device of the present invention does not include a rotator, and no interfering signal is obtained at two or more rotational orientations of the polarization axis of the sample optical signal. For example, if the polarization axis of the sample optical signal is not along the optical axis of the phase delay generator including the variable retarder, no rotator is needed. However, if the polarization axis of the sample optical signal is actually substantially linearly polarized along the optical axis of the phase delay generator, no phase delay is added between the two polarization components of the sample optical signal, and this problem May be added before the phase delay generator to address
[0061]
Referring again to FIGS. 3A and 3B, in one embodiment, the sampled optical signal is sent from the rotator 302 to phase delay generators 304, 354, which include one polarization component of the sampled optical signal. To the other. Delaying one polarization component relative to the other can be achieved, for example, by propagating the polarization component through different lengths of light path or through a variable birefringent element. In various embodiments, the phase delay generator includes a variable birefringent element 304. Suitable variable birefringent elements include, but are not limited to, variable retarders. Preferably, the variable retarder comprises an electro-optical and / or liquid crystal wave plate. In other embodiments, the phase delay generator includes elements that propagate the polarization components through different lengths of the optical path.
[0062]
Referring to FIG. 3B, in one embodiment, the phase delay generator 354 includes a polarization splitting beam splitter 355 and a variable delay element 357. The sample optical signal is then split into two beams by a polarizing beam splitter (“PBS”) 355 and one of the polarization components (eg, E_x) Is transmitted and the other (eg, E_y) Is reflected. A quarter wave plate and reflector 359 in each arm rotate the polarization and recombine the two beams after the beam splitter. A variable phase delay is introduced into one of the separated beams (ie, polarization components) by a variable delay element 357 that creates a variable retardance between the two propagating optical paths. Possible embodiments of the variable delay element include, but are not limited to, a fixed axis liquid crystal retarder, a variable retardance wave plate, and a spatially variable mirror located behind at least one quarter wave plate.
[0063]
Referring to FIG. 4, a preferred embodiment of the rotator 402 and the phase delay generator 404 is shown. Preferably, rotator 402 includes a first switchable half-wave plate 403 and a second switchable half-wave plate 405, and phase delay generator 404 includes a fast-wave axis 413 of half-wave plates 403 and 405, 415 includes a variable retarder having a fast wave axis aligned in a fixed orientation with respect to 415. In one embodiment, the direction of the fast wave axis of the second half-wave plate 405 is rotated by φ / 2 (425) with respect to the fast wave axis of the first half-wave plate 403, and the polarization axis of the sample optical signal is rotated. Is changed by φ with respect to the optical axis of the phase delay generator 404. The orientation can then be reset by changing the retardance of the half-wave plate to zero.
[0064]
Referring again to FIGS. 3A and 3B, the sampled optical signal is sent from phase delay generators 304, 354 to interferometer 306. Interferometer 306 mixes the orthogonal polarization components of the sample optical signal under conditions that can cause interference between them, thereby generating an interfering signal. Preferably, the interferometer includes a 45 ° linear analyzer. However, as will be appreciated, any optical element or configuration of optical elements that can recombine the polarization components of an optical signal under conditions that cause interference therebetween functions as an interferometer in the polarization monitoring device of the present invention. can do.
[0065]
In various embodiments of the present invention, the interference signal generated by the interferometer is measured by a detector. In various other embodiments, the interfering signal is dispersed into spectrally contiguous wavelength subbands, and different detector elements receive the interfering signal corresponding to a particular wavelength channel of the optical transmission system. The detector elements can consist of an array of detectors, for example, two or more physically independent detectors, or an integrated detector array. Examples of suitable detector arrays include, but are not limited to, an array of charge coupled devices ("CCD"), an array of integrated photodiodes, and an array of separate detectors. In telecommunications applications, InGaAs photodiodes are suitable for array and separate detector applications. However, as used herein and in the claims, as will be understood, the terms "array of detectors" and "detector array" may be used interchangeably. That is, when an element is described by the term "array of detectors", as will be understood, the term also includes "detector array" and vice versa.
[0066]
3A and 3B, in one embodiment, the interference signal generated by interferometer 306 is a wavelength demultiplexer 308 that spectrally disperses the sampled optical signal into spectrally continuous wavelength sub-bands. Pass through. As a result, splitter 308 provides an interference signal for each wavelength sub-band. In one embodiment, each wavelength channel of an optical signal (eg, a DWDM signal, etc.) is split by spectrally dispersing it into at least two wavelength sub-bands. Preferably, the wavelength demultiplexer divides each wavelength channel into 5 to 15 wavelength sub-bands. Examples of suitable wavelength demultiplexers include, but are not limited to, an arrayed waveguide grating (AWG), a volume phase grating spectrometer, and a reflection grating spectrometer in free space and planar implementations. included.
[0067]
By spectrally dispersing the interference signal into interference signals of each wavelength sub-band, it becomes easy to measure the PSP characteristics of the multiple wavelength channels of the optical signal. Further, spectrally dispersing the interference signals across the detector array allows the multi-channel interference signals to be measured substantially simultaneously. For example, in one embodiment, wavelength splitter 308 spectrally disperses the interfering signal into spectrally contiguous subbands across detector array 310. Then, different sets of elements of the detector array 310 receive substantially simultaneously interfering signals corresponding to different wavelength channels. As a result, the interference signals of multiple wavelength channels can be measured substantially simultaneously and this information is used by the polarization state generator to determine the relative amplitude and polarization state of the PSP and the DGD for the multiple wavelength channels.
[0068]
The polarization state generator can consist of analog and / or digital devices. The polarization state generator determines the polarization state and relative amplitude of the PSP of the optical signal and the DGD between them. In one embodiment, the polarization state generator determines a PSP characteristic based on the measured interference signals for a plurality of phase delays in each of the two or more rotational orientations of the sampled optical signal. Further, the polarization state generator can determine the PSP characteristics of the multiple wavelength channels substantially simultaneously based on the measured interference signals for a plurality of phase delays in each of one or more rotation orientations of the sampled optical signal. . Preferably, the polarization state generator determines the PSP characteristics of the multiple wavelength channels also based on the relationship between the interference signals observed for different wavelength sub-bands in different wavelength channels.
[0069]
In one embodiment, the polarization state generator determines PSP characteristics using logic substantially according to equations (3)-(6). In another embodiment, the polarization state generator determines PSP characteristics using logic substantially according to equations (7)-(19). However, as will be appreciated, the polarization state generator may include any logic suitable for determining a PSP characteristic based on a plurality of phase-delayed interference signals in each of one or more rotational orientations of the sampled optical signal. Can be used.
[0070]
The polarization state generator logic may be implemented by analog, digital, and / or software on a general-purpose computer. The polarization state generator can produce a human readable representation of the relative amplitude and polarization state of the PSP, and the DGD between them, for example, as generated by a printer or computer screen. However, whether the polarization state generator produces only human-readable or machine-readable output is not critical to the present invention. For example, the polarization state generator may generate only enough machine-readable electrical signals to control a polarization controller that compensates for the PMD of the optical transmission system.
[0071]
As will be appreciated, various embodiments of the polarization monitoring device of the present invention can be used as a multi-channel Stokes vector polarimeter because the required polarization characteristics are sufficient to calculate the Stokes vector of light in any wavelength channel. Suitable for use. Also, embodiments of the polarization monitor are described as multi-channel power monitors because the average intensity on one or more detector elements illuminated by the spectrum of one wavelength channel results in a measurement of the average power of the channel. Also suitable for use. Finally, an embodiment of the polarization monitor is used as a multi-channel spectrum analyzer because the average intensity of the detector channel of each monitor results in a measurement of the power in the spectral bin corresponding to the detector size and the dispersion of the grating. obtain.
[0072]
In one embodiment, the present invention provides an apparatus for compensating PMD of an optical signal in an optical transmission system. Referring to FIGS. 5 and 6, various embodiments of the PMD compensator are shown. The compensator includes polarization monitors 503 and 603, polarization state generators 517 and 617, and compensation stages 521 and 621. The polarization monitoring device and the compensation stage can include any of the embodiments described herein. As illustrated, the polarization monitor 503 of FIG. 5 is substantially the same as that shown in FIG. 3A and the accompanying description, and the polarization monitor 603 of FIG. 6 is substantially the same as that shown in FIG. 3B and the accompanying description. Substantially the same as shown.
[0073]
In operation, the polarization monitoring devices 503, 603 provide the polarization state generators 517, 617 with measurements of the interference signal at a plurality of phase delays for each of one or more rotation orientations of the sampled optical signal. The polarization state generators 517, 617 then determine the PSP characteristics of the optical signal, or one or more wavelength channels of the optical signal, based on the measurements provided by the polarization monitoring devices 503, 603. In one embodiment, the polarization state generator 517, 617 provides a control signal to a polarization controller driver 530, 630 that provides a control signal to the polarization controller 522, 622 of the compensation stage 521, 621, and then provides a control signal to the compensation stage 521. , 621 add DGD to the optical signal in a selectable orientation to transfer energy of the optical signal to substantially one PSP of the optical transmission system. Alternatively, as described above, the compensation stage may be considered as modifying the PSP of the optical transmission system such that the polarization state of one PSP is substantially the same as that of the optical signal.
[0074]
Referring again to FIGS. 5 and 6, in various embodiments, the compensation stage includes polarization controllers 522, 622 and delay elements 524, 624. Preferably, the delay elements 524, 624 have a substantially fixed DGD that is larger than the DGD of the optical transmission system associated with the PMD to be compensated. Thus, if the delay elements 524, 624 have a substantially fixed DGD, the polarization controller drivers 530, 630 provide only the control signals to the polarization controllers 522, 622. However, as will be appreciated, polarization controller drivers 530, 630 and / or polarization state generators 517, 617 are not limited to sensors (eg, temperature, position, power, etc.), but may include system diagnostics and control systems, systems. Input signals can be received from various sources, including clocks and the like, to facilitate PMD compensation. For example, the DGD of the delay elements 524, 624 varies with temperature, so that the DGD of the delay element can be determined using the input signal from the delay element temperature sensor.
[0075]
As illustrated in FIGS. 5 and 6, the compensation stages 521, 621 are adapted to compensate for PMD in multiple wavelength channels of an optical transmission system. In the illustrated embodiment, the compensation stages 521, 621 comprise wavelength splitters 526, 626, polarization controllers 522, 622, wavelength multiplexers (in the order of the optical path of the general optical signal from source to receiver). 528, 628 and delay elements 524, 624. Wavelength demultiplexers 526, 626 disperse the optical signal into spectrally continuous wavelength sub-bands, as represented by lines 551, 651. The polarization control device controls each wavelength channel such that the DGD of the delay elements 524, 624 is added in an orientation such that a sufficient fraction of the energy of the wavelength channel is within a single PSP of the corresponding wavelength channel. Works with Generally, the polarization controller adds DGD at different orientations of each wavelength channel. As a result, the wavelength combiner recombines the light (represented by lines 553, 653) operated by the polarization controller.
[0076]
As illustrated in FIGS. 5 and 6, compensation stages 521, 621 mean that the optical path of the optical signal traveling from points 501, 601 to points 599, 699 passes through the compensation stages before encountering the optical monitoring device. , Prior to the polarization monitoring devices 503 and 603. It should be understood, however, that the placement of the compensation stage "before" or "after" the polarization monitor is not critical to the invention. For example, if the polarization monitor precedes the polarization controller, the Jones or Mueller matrix of the polarization controller can be used to convert the polarization state between the two ends of the polarization controller.
[0077]
The compensation stages useful in the present invention can take various forms. Preferably, the delay element of the controller comprises a polarization preserving fiber ("PMF"). However, suitable delay elements include, but are not limited to, free space and fiber delay elements, or birefringent crystals such as calcite or yttrium vanadate. Suitable wavelength multiplexers and demultiplexers may include gratings 307, 537, 637 and dispersion collimators 309, 539, 639, as illustrated in FIGS. 3A, 3B, 5, and 6. it can. Further suitable wavelength demultiplexers and multiplexers include, but are not limited to, in free space and planar implementations, an arrayed waveguide grating (AWG), a volume phase grating spectrometer, and a reflection grating spectrometer. Also, a suitable polarization controller can take various forms. For example, a polarization control device can include a series of variable wave plates, lithium niobate wave plates, liquid crystals, fiber squeezers, and silica-based stress retarders composed of electro-optic crystals.
[0078]
Preferably, the compensation stage includes a delay element having a substantially fixed DGD and an LC polarization controller. Referring to FIG. 7, a preferred embodiment of the LC polarization controller is shown. LC polarization controller 701 includes a stack of four LC waveplates 702, 704, 706, and 708. Preferably, the stack of LC waveplates is less than 10 mm thick, and preferably the absolute retardance error of the LC waveplate is no more than 4 nm per waveplate.
[0079]
Preferably, each LC waveplate includes an array of LC cells 710 housed between optical quality glasses coated with a coating of conductive indium tin oxide (ITO). Each individual LC cell functions as a tunable waveplate substantially from 0 to 1.2 waves in the wavelength band of interest and can be controlled by individual ITO electrodes. The LC cell is preferably electrically controlled with a response time of less than 3 ms. For example, the response can be temporarily shortened by raising the temperature of the element.
[0080]
The four wave plates 702, 704, 706, and 708 are preferably configured to nominally have their rubbing directions at 0 °, 45 °, 0 °, and 45 °, respectively, Are laminated together with an optically transparent epoxy of refractive index matched to the wavelength region of interest. Preferably, cell alignment (in the xy direction) is more than 25 μm between the first LC waveplate 702 and the last LC waveplate 708. Further, the LC polarization controller may further include a front integrated lens assembly 712 and / or a rear integrated lens assembly 714.
[0081]
Preferably, the individual LC cells are sized such that the demultiplexer of each communication channel is greater than 90% of the spacing of each ITU grid after transmitting through the active area of each cell. In other words, less than 10% of the bandwidth between ITU channels is preferably lost from the intercell gap 716 between LC cells.
[0082]
In some embodiments, the functions of the above-described method may be implemented as software on a general-purpose computer. Further, such a program may be stored in a portion of the computer's random access memory to control the rotator, control the phase delay generator, control the interferometer, measure the interference signal, control the polarization controller, and measure the measured interference signal. , And control logic that affects the operation with respect to the interference signal. In such embodiments, the program may be written in any one of a number of high-level languages, such as Fortran, Pascal, C, C ++, or Basic. In addition, programs can be written with scripts, macros, or functions built into commercial software such as Excel or Visual Basic. Further, the software may be implemented in assembly language for a computer resident microprocessor. For example, if the software was configured to run on an IBM PC or PC clone, it could be implemented in Intel 80x86 assembly language. The software may be incorporated into an article of manufacture including, but not limited to, a "computer-readable medium" such as a floppy disk, hard disk, optical disk, magnetic tape, PROM, EPROM, or CD-ROM.
[0083]
Example of polarization monitoring and PMD compensation
Referring to FIGS. 5 and 8, an example of an embodiment for monitoring and compensating for PMD in an optical transmission system is as follows. The sample optical signal is branched from the optical transmission system by the 90/10 optical tap 502 and input to the polarization monitor 503 via the optical coupler 504. The polarization monitor is cooled by a polarization rotator 506, a phase delay generator 508 consisting of a variable retarder, an interferometer 510 consisting of a 45 ° linear polarizer, a wavelength demultiplexer 512, and multi-element thermoelectron (“TE”) cooling. An array 514 of detectors comprising an InGaAs array is included.
[0084]
In this example of the polarization monitor configuration of FIG. 5, the order of data collection for a single update period is as follows. That is,
(1) The polarization axis of the sample optical signal is angle φ₁The polarization rotator 506 is set to rotate by (for example, 0 °).
(2) change the phase delay (eg, dither phase delay) provided by the variable retarder 508, for example, through:
a. A continuous periodic retardance (phase delay) characteristic, such as a sinusoidal change of a 0-1 wave, or
b. Several separate retardance (phase delay) steps.
(3) During step 2, the detector array 514 measures the interference signal in a spectrally continuous wavelength band provided by the wavelength demultiplexer 512 for each rotator-retarder setting.
(4) The polarization axis of the sample optical signal is angle φ₂The polarization rotator 506 is set to rotate by (for example, 45 °).
(5) Steps 2 and 3 are repeated.
Alternatively, the polarization rotator can dither the sample optical signal between the two rotation orientations so that the phase delay is changed or stepped slowly.
[0085]
The data collection control signal for this example is shown schematically in FIG. The polarization rotator operates with a 50% duty cycle, i.e., substantially equal times, with two rotation orientations φ as indicated by the rotor control signal pattern 801.₁811 and φ₂Spent at 812. At each polarization rotator position, i.e., at each rotational orientation of the sample optical signal, the phase is dithered by a phase delay generator, resulting in a plurality of phase delays between the polarization components of the sample optical signal within the phase dither time window 831. The phase dither can be a phase delay or a continuous temporal variation of a series of phase delay steps. The phase delay can be changed throughout the wave or a portion thereof. In this example, as shown by the phase delay generator control signal pattern 803, the phase dither includes four phase delay steps between one full wave λ and zero during a time window 831.
[0086]
With reference to FIGS. 5 and 8, during data acquisition, the phase delayed sampled optical signal passes through interferometer 510 and is spectrally dispersed onto detector array 514 with wavelength demultiplexer 512. . The detector array measures the signals distributed over the wavelength channels, each channel being in a different set of elements of the array. Thus, the detector array measures the interfering signal substantially simultaneously for all of the wavelength channels. The data acquisition pattern 802 of the detector array indicates the timing 821 of detector measurement of the interference signal (ie, detector exposure) relative to the amount of change in the polarization rotator and phase delay generator. The phase delay may be kept constant over each detector exposure or may be ramped.
[0087]
The polarization state generator 517 processes all interference signals in wavelength sub-bands that span the desired spectral width of the optical signal. The polarization state generator processing pattern 804 indicates the timing 841 of the interference signal measurement processing for another data collection operation. The polarization state generator determines the PSP characteristics, and based on this information, the polarization control vector is determined for a wavelength channel that spans the desired spectral width of the optical signal. The control vector measurement pattern 806 illustrates the timing 861 of the polarization control vector measurement for other data collection and processing operations. In this example, the polarization control vector is determined from the measured intensities of the interference signals of each wavelength channel substantially according to equations (7)-(12) and (13)-(19).
[0088]
Based on the polarization control vector, polarization controller driver 530 determines the drive signal for polarization controller 522, which determines a sufficient fraction of the optical signal energy for each wavelength channel into a single PSP for the channel. Move to Polarization controller driver pattern 808 indicates the timing 881 of the application of the polarization controller drive signal to other data collection and processing operations.
[0089]
In a preferred embodiment, the compensation stage compensates for the PMD of the optical signal as follows. Initially, the signal is spectrally dispersed by wavelength splitter 526, with one wavelength channel passing through each channel (ie, set of elements) of multi-channel polarization controller 522. The light of that wavelength channel is then recombined by wavelength combiner 528 and sent to a delay element consisting of a single polarization-maintaining fiber 524 having a DGD greater than the amount of DGD to be compensated. The polarization controller 522 changes the polarization state of each channel so that the polarization state of the optical signal matches the PSP of the combination of the transmission system and the PMD compensator. Preferably, the sample optical signal of the polarization monitor 503 is split off from the transmission system after the polarization controller so that a feedback signal and a diagnostic signal of the polarization controller can be determined. It should be understood, however, that the feedback and / or repetition control of the polarization controller, as described above, is not critical to the present invention.
[0090]
As shown in FIG. 8, the time interval from the start of data collection to the completion of PMD compensation, that is, the update cycle 807 in this example is 8 ms. The time pattern 805 indicates that about 3 ms of this time is spent collecting data, and about 2 ms during that data collection to generate interference signals at multiple phase delays and rotation orientations in the first detector exposure 851. About 1 ms is spent generating an interference signal at a plurality of phase delays and rotational orientations at the second detector exposure 852. Further, in one embodiment, for each detector exposure period 851, 852, at least six measurements 855, 856 are made. As illustrated, for the first detector exposure period 851, the measurements 855 consist of substantially equal durations (eg, each measurement takes 2 ms of the first detector exposure period 851 and six measurements 855). About 1/3 of the length of 1 ms). Similarly, for the second detector exposure period 852, the measurements 856 are of substantially equal duration (eg, each measurement is 1 ms relative to the second detector exposure period 852 and 1 ms for the six measurements 856). About 1/6 of the length).
[0091]
The time pattern 805 is that approximately 2 ms of the update cycle in this example is spent processing the data and / or ramping 853 of the polarization rotator and phase delay generator, and approximately 3 ms driving the polarization controller 522 and the optical signal. It is further shown that PMD is spent on PMD compensation. In certain applications, the process of data collection and PMD compensation is repeated to account for changes in the degree of PMD and / or changes in the PSP of the transmission system over a period of time. In this example, the PMD compensation cycle provided by the present invention operates at a frequency of 125 Hz.
[0092]
While the invention has been particularly shown and described with respect to particular embodiments, it will be understood by those skilled in the art from the spirit and scope of the invention as defined by the appended claims. Various changes in form and detail may be made without departing from the invention. The scope of the invention is, therefore, indicated by the appended claims and, therefore, all modifications that come within the meaning and range of equivalents of the claims are intended to be embraced.
[Brief description of the drawings]
[0093]
FIG. 1 is a diagram illustrating a PMD vector of an optical transmission system and a PSP represented by a Poincare sphere.
FIG. 2A is a diagram illustrating one embodiment of a PMD compensation method of the present invention.
FIG. 2B is a diagram showing one embodiment of a PMD compensation method of the present invention.
FIG. 3A is a schematic diagram illustrating various embodiments of the polarization monitoring device of the present invention.
FIG. 3B is a schematic diagram illustrating various embodiments of the polarization monitoring device of the present invention.
FIG. 4 is a schematic diagram illustrating one embodiment of a rotor and a phase delay device of the present invention.
FIG. 5 is a schematic diagram showing one embodiment of a PMD compensator of the present invention.
FIG. 6 is a schematic diagram illustrating one embodiment of a PMD compensator of the present invention.
FIG. 7 is a schematic diagram illustrating one embodiment of a polarization controller of the compensation stage of the present invention.
FIG. 8 is a schematic diagram illustrating an embodiment of the polarization monitoring method of the present invention.

Claims (43)

A method for compensating for polarization mode dispersion of an optical signal,
Providing a sample optical signal;
Causing the first polarization component and the second polarization component to interfere with each other for a plurality of phase delays between the first polarization component and the second polarization component of the sample optical signal; and Compensating the polarization mode dispersion of the optical signal based on the information. Compensating the polarization mode dispersion,
Determining the polarization state of the optical signal using the measured intensity of the interference signal;
Determining a polarization mode dispersion vector of the optical signal using the measured intensity of the interference signal; and determining a modification to the optical signal that substantially compensates for the polarization mode dispersion of the optical signal. Using the polarization state of the polarization state and the polarization mode dispersion vector. 3. The method of claim 2, wherein the modification to the optical signal transfers a sufficient portion of the energy of the optical signal to a single dominant polarization state of the optical transmission medium. Determining the polarization state of the optical signal,
Associating the measured intensity of the interference signal for a first rotational orientation of the polarization component with a first sinusoidal function that is a function of the phase delay;
Associating the measured intensity of the interference signal for the second rotation orientation of the polarization component with a second sinusoidal function that is a function of the phase delay; and Determining the state of polarization of the optical signal by solving for a phase offset between orthogonal polarization components. The method of claim 4, further comprising determining a Stokes vector of the optical signal. The method of claim 4, further comprising determining a Jones vector of the optical signal. Causing interference with the polarized light component,
Using a phase delay generator, for each of at least two rotational orientations of the polarization axis of the sample optical signal with respect to the optical axis of the phase delay generator, between the first polarization component and the second polarization component; Introducing at least two phase delays;
Providing an interference signal of each phase delay in each rotation direction by causing interference between the first and second polarization components, and measuring the strength of each of the interference signals. Item 1. The method of Item 1. The method of claim 7, wherein compensating for the polarization mode dispersion comprises compensating for the polarization mode dispersion of the optical signal based on the measured strength of the interference signal. Compensating the polarization mode dispersion of the optical signal,
Spectrally dispersing the interference signal into spectrally contiguous sub-bands and measuring substantially simultaneously the intensities of the interference signals in two or more of the spectrally contiguous sub-bands. The method of claim 7, comprising: 10. The method of claim 9, wherein compensating for the polarization mode dispersion of the optical signal comprises compensating for substantially simultaneously the polarization mode dispersion of the channels of the two or more spectrally dispersed optical signals. Method. An article of manufacture having a computer readable medium comprising computer readable instructions embodied to perform the method of claim 1. A method for compensating for polarization mode dispersion of an optical signal,
Providing a sample optical signal;
Introducing at least three phase delays between a first polarization component and a second polarization component of the sample optical signal;
Providing interference signals of each phase delay by causing interference between the first and second polarization components;
Measuring a strength of each of the interference signals; and compensating polarization mode dispersion of the optical signal based on the measured strength of the interference signals. Compensating the polarization mode dispersion,
Determining the polarization state of the optical signal using the measured intensity of the interference signal;
Determining a polarization mode dispersion vector of the optical signal using the measured intensity of the interference signal; and determining a modification to the optical signal that substantially compensates for the polarization mode dispersion of the optical signal. Using the polarization mode dispersion vector and the polarization mode dispersion vector. 13. The method of claim 12, wherein the modification to the optical signal transfers a sufficient fraction of the energy of the optical signal to a single dominant polarization state of the optical transmission medium. Determining the polarization state of the optical signal,
Associating the measured intensity of the interference signal for a first rotational orientation of the polarization component with a first sinusoidal function that is a function of the phase delay;
Associating the measured intensity of the interference signal for the second rotation orientation of the polarization component with a second sinusoidal function that is a function of the phase delay; and Solving for the phase offset between orthogonal polarization components to determine the polarization state of the optical signal. The method of claim 15, further comprising determining a Stokes vector of the optical signal. The method of claim 15, further comprising determining a Jones vector of the optical signal. Measuring the strength of the interference signal,
Spectrally dispersing the interference signal into spectrally contiguous sub-bands and measuring substantially simultaneously the intensities of the interference signals in two or more of the spectrally contiguous sub-bands. 13. The method of claim 12, comprising: 19. The method of claim 18, wherein compensating for the polarization mode dispersion of the optical signal comprises compensating for substantially simultaneously the polarization mode dispersion of the channels of the two or more spectrally dispersed optical signals. Method. An article of manufacture having a computer readable medium comprising computer readable instructions embodied to perform the method of claim 12. An apparatus for determining polarization mode dispersion of an optical signal,
A phase delay generator arranged to receive a sample optical signal that includes at least a portion of the optical signal;
An interferometer that is in optical communication with the phase delay generator and arranged to cause interference with a polarization component of the sample optical signal received from the phase delay generator to generate an interference signal;
A detector positioned in optical communication with the interferometer and arranged to measure the interference signal; and a polarization state generator for determining a polarization state of the optical signal based on the plurality of measured interference signals. ,apparatus. 22. The apparatus of claim 21, further comprising a rotator arranged to provide the phase delay generator with at least two rotational orientations of a polarization axis of the sample optical signal with respect to an optical axis of the phase delay generator. 23. The apparatus of claim 22, wherein said rotator comprises a polarization rotator adapted to rotate a polarization axis of said sample optical signal. 24. The apparatus of claim 23, wherein said polarization rotator comprises a Faraday rotator. 24. The apparatus of claim 23, wherein said polarization rotator comprises a series of two or more wave plates. 23. The apparatus of claim 22, wherein the rotator comprises a phase delay generator rotator adapted to rotate the optical axis of the phase delay generator with respect to the polarization axis of the sample optical signal. 22. The apparatus of claim 21, wherein said phase delay generator comprises a variable retarder. 22. The apparatus of claim 21, wherein said interferometer comprises a Michelson interferometer. 22. The apparatus of claim 21, wherein said interferometer comprises a 45 degree linear polarizer. 22. The apparatus of claim 21, further comprising a wavelength demultiplexer in optical communication with the interferometer and arranged to spectrally disperse the interference signal into spectrally continuous subbands. 31. The apparatus of claim 30, wherein the detectors comprise an array of detectors, each detector of the array being arranged to measure an interference signal in one spectrally continuous sub-band. The optical fiber of claim 1, further comprising a compensation stage in optical communication with the optical transmission medium and adapted to substantially compensate polarization mode dispersion of the optical signal in the optical transmission medium based on a polarization state of the optical signal. 21 devices. The compensation stage comprises:
A wavelength demultiplexer optically communicating with the optical transmission medium and arranged to spectrally disperse the optical signal into spectrally dispersed channels;
An array of polarization controllers arranged in an optical path between the wavelength demultiplexer and a wavelength multiplexer optically communicating with the optical transmission medium,
33. The apparatus of claim 32, wherein the array of polarization controllers is adapted to substantially compensate for polarization mode dispersion of each of the spectrally dispersed channels. 34. The apparatus of claim 33, wherein the array of polarization controllers includes a plurality of liquid crystal variable retarders. An apparatus for compensating for polarization mode dispersion of an optical signal,
A phase delay generator arranged in the optical transmission medium to receive a sample optical signal including at least a portion of the optical signal;
A rotator arranged to provide at least two rotational orientations of the polarization axis of the sample optical signal with respect to the optical axis of the phase delay generator;
An interferometer arranged to cause interference with a polarization component of the sample optical signal received from the phase delay generator to generate an interference signal;
A wavelength demultiplexer arranged to spectrally disperse the interference signal into spectrally continuous sub-bands,
An array of detectors, wherein each detector of the array is arranged to measure an interference signal in one spectrally continuous sub-band;
A polarization state generator that determines a polarization state of an optical signal for each of the spectrally continuous bands based on a plurality of measured interference signals, and optically communicates with the optical transmission medium; A compensation stage adapted to compensate for the polarization mode dispersion of the optical signal in each of the spectrally dispersed channels based on the polarization state of the spectrally continuous sub-bands of the dispersed channels. ,apparatus. 36. The device of claim 35, wherein the polarization control device includes a plurality of liquid crystal variable retarders. The polarization controller,
A wavelength demultiplexer optically communicating with the optical transmission medium and arranged to spectrally disperse the optical signal into spectrally dispersed channels;
An array of polarization controllers arranged in an optical path between the wavelength demultiplexer and a wavelength multiplexer optically communicating with the optical transmission medium,
36. The apparatus of claim 35, wherein the array of polarization controllers is adapted to substantially compensate for polarization mode dispersion of each of the spectrally dispersed channels. 36. The apparatus of claim 35, wherein the rotator comprises a polarization rotator adapted to rotate a polarization axis of the sample light signal. 36. The apparatus of claim 35, wherein said phase delay generator comprises a variable retarder. 36. The apparatus of claim 35, wherein said interferometer comprises a Michelson interferometer. 36. The apparatus of claim 35, wherein said interferometer comprises a 45 degree linear polarizer. A method for compensating for polarization mode dispersion of an optical signal in an optical transmission medium,
Providing a sample optical signal that includes a portion of the optical signal of the optical transmission medium;
Introducing a first phase delay between a first polarization component and a second polarization component of the sample optical signal and interfering with the first and second polarization components to generate a first interference signal The step of causing
Measuring the strength of the first interference signal;
Introducing a second phase delay between a first polarization component and a second polarization component of the sample optical signal and interfering with the first and second polarization components to generate a second interference signal The step of causing
Measuring the strength of the second interference signal;
A third phase delay is introduced between a first polarization component and a second polarization component of the sample optical signal to interfere with the first and second polarization components to generate a third interference signal. The step of causing
Measuring the strength of the third interference signal;
Providing a rotated sample optical signal by rotating the polarization axis of the sample optical signal;
Introducing a fourth phase delay between a first polarization component and a second polarization component of the rotated sample optical signal and generating the fourth interference signal with the first and second polarizations. Causing the components to interfere;
Measuring the strength of the fourth interference signal;
Introducing a fifth phase delay between a first polarization component and a second polarization component of the rotated sample optical signal to produce a fifth interference signal. Causing the components to interfere;
Measuring the strength of the fifth interference signal;
The first and second polarizations to introduce a sixth phase delay between a first polarization component and a second polarization component of the rotated sample optical signal to generate a sixth interference signal. Causing the components to interfere;
Measuring the strength of the sixth interference signal; and determining the polarization of the optical signal based on the measured strengths of the first, second, third, fourth, fifth, and sixth interference signals. Compensating for wave mode dispersion. 43. An article of manufacture having a computer readable medium comprising computer readable instructions embodied to perform the method of claim 42.

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